star wars - Why is the clone army so...small?

The number of Republic and/or Separatist planets isn't what's important -- it's the number of droids faced by the clone army. If there was no Separatist Droid Army then 200,000 clones would be more than enough for the Republic to force its will upon the Separatists. Conversely, had the Senate continued to be deadlocked over the Military Creation Act and the clone army hadn't already existed, the Separatists could have have easily defeated the Republic with its droid army. Imposing its will across the entire galaxy would be difficult for either the Republic or Separatists (the Republic struggled to control the outer regions of the galaxy even in peacetime) but whichever side had an unopposed army would have been able to depose its opponent's seat of government (e.g. Coruscant).

Wookieepedia's Legends article claims that there were 1 million B1 battle droids, 100,000 B2 battle droids, and 3,000 droidekas at the first Battle of Geonosis, but these figures are not well sourced. Assuming they are true, however, that puts the clone army at about a 5-to-1 disadvantage. Nonetheless, the clones had the element of surprise and over two hundred Jedi on their side. Furthermore, the clones are touted as much better than droids:

They're immensely superior to droids, capable of independent thought and action.

_{Lama Su, Star Wars: Episode II Attack of the Clones}

These advantages were apparently enough to overcome the clone army's numerical disadvantage since the clone army literally routed the droid army at the first Battle of Geonosis (in that the Separatists fled in disorder).

Building droids takes less time than growing clones, so one would think that the Separatists' numerical advantage would increase over time. However, the Separatists were not prepared for full-scale production at the beginning of the Clone Wars since the Separatists didn't even know of the existence of the clone army. Furthermore, the clones are age-accelerated so that they only take half the normal time to reach adulthood. And, again, don't forget the huge advantage offered by the Jedi.