The larger the pool, the more it destabilizes the ship as the water moves. Smaller pools mean less water movement, and it's easier to install slosh-reducing features in them.
A user on the subreddit r/cruise explained that if pools were located in the lower parts of the ship, they could be larger, but no one wants to swim in a dark, below-deck pool.
If you're scared of capsizing, consider taking a river cruise, as their ships often lack pools. However, even with a pool, the chances of a cruise ship tipping over are minuscule.