Recall that under Solon all free, native-born Athenian men, however poor, became citizens. But they were not equal citizens. Although distinctions of birth were abolished among citizens, they were grouped into four classes by wealth. Although all could serve in the assembly or on juries, only the first three classes (i.e., property holders) could belong to the Council. And only members of the first class (by some accounts) or the first two classes (the cavalry, by other accounts) could hold the archonship. The nine archons appear to have been powerful officials, holding the executive power of the state. And there was a body of ex-archons, serving for life known as the Areopagus. Its function is somewhat unclear, but Aristotle says:
The Council of Areopagus had as its constitutionally assigned duty the protection of the laws; but in point of fact it administered the greater and most important part of the government of the state, and inflicted personal punishments and fines summarily upon all who misbehaved themselves.In other words, Solon's government was moderately tight. Poor men could only vote and serve on juries, not hold office. And the bulk of power probably rested with the nine archons and lifetime body of ex-archons. But it was also broad. Solon restored citizenship to anyone disenfranchised before him, except for treason or murder. He offered citizenship to any immigrant prepared to move to Athens with his family and conduct a useful trade. So it would appear that if Solon had had his way, there would have been no permanent class of metics, or unnaturalized immigrants. As for slaves, we have no idea of knowing how many there were. However, Athens was poorer and less commercial than it would become, so in all probability there were fewer. (Herodotus mentions that at this time many families did not have household slaves and instead sent their daughters to fetch water). In short, in all probability citizens made up a larger share of the population, and citizenship was easier to get in Solon's day than it would be at any time in the future.
When Cleisthenes restored the democracy, he appears to have resolved disputes over who was and was not a legitimate citizen by the simple expedient of granting citizenship to all free, native-born residents but saying that in the future, a citizen must have an Athenian father. In other words, he made government temporarily broader, but ensured that it would narrow over time. Aristotle says that Cleisthenes made the government more democratic than before, which appears to mean that he made it more diffuse. For instance, while the archon polemarch was originally the supreme commander-in-chief, Cleisthenes replaced him with ten generals (one from each tribe). By Aristotle's time, the polemarch apparently had no command at all and was simply responsible for all religious ceremonies relating to war. Presumably other archons started seeing their power restricted and spread in similar fashion. (Again, see Aristotle for how little importance these once-powerful officials had left).
The next great diffusion in power came with the Persian Wars. The four rights of first-class citizenship, from ancient times to the present, are the right to vote, to hold office, to sit on juries and to serve in the military. Poor Athenians since Solon's time had had the right to vote and to sit on juries but were barred from holding office. As for military service, poor men were not barred outright, but each soldier was required to supply his own gear. Poor men who could not afford gear (beyond, perhaps, a sling), could not serve. And military importance was closely linked with political power. This began to change between the famous Battle of Marathon, which took place a mere twenty years after democracy was restored, and the bulk of the Persian War ten years later. A leader named Themistocles, of humble and obscure origin, and perhaps something of a rough and ready populist, called for building a navy to meet the Persians if they returned. Although the debate was formally over how best to defend the city, the political implications were clear. Men too poor to serve in the army could serve as sailors* and earn the political power that comes with military importance. Put differently, it is well known that Athens was both a democracy and the supreme naval power of its day. Less well understood were how closely linked democracy and naval power were. Athens' reliance on naval power successfully defeated the Persians. As a reward to the efforts of Athens' poor citizens, their restrictions on office were lifted.
Finally, radical democrats Ephialtes and Pericles diminished the powers of the Areopagus (the body of ex-archons serving for life) to trials of murder and some religious matters. Classical authorities are unanimous in condemning this action, saying that it removed the last brake on the democracy and allowed the people to run wild. Unfortunately, none of them are very clear on what the Areopagus' functions were** or precisely what happened as a result of cutting back on its power, so any modern opinion on the subject cannot be much more than guesswork.
Then there was the matter of holding office. Although removing the means test removed any formal barrier to poor men holding office, the amount of work involved was a full-time job, which effectively barred office to anyone who couldn't afford to take a year off of work without pay. The democrats therefore introduced pay (quite modest) for office holding. Pericles also introduced pay (even more modest) for jury service to make it easier for poor men.
But if Athenian democracy became steadily more diffuse, it also became narrower. In Solon's day, any Greek moving to Athens and settling there permanently with his whole family to pursue a useful trade could become a citizen. As the numbers of these immigrants swelled, the rule changed to allow only children of an Athenian father to become citizens. Still, this allowed metics (especially wealthy ones) to marry their daughters to citizens and have citizen grand children. Even an Athenian man's children by his slaves might become citizens. Many of Athens' greatest statesmen in its early days had non-citizen mothers. But Pericles changes the law to require both parents to be citizens for their children to be citizens. According to Plutarch, some 5,000 people were convicted of falsifying citizenship and sold as slaves. This was a major narrowing that caused the citizen body to dwindle over time. It also seems a fair assumption that, as Athens became richer and more commercial, the slave population swelled.
Thus the assumption that Athens simply became more democratic over time is an over simplification, at least from the modern perspective.
*Technically, as rowers, but that raises certain unfortunate -- and false -- associations. So I will stick with the admitted euphemism "sailors."
**Although they apparently included trials for treason.