For Hexcells being the first game it has very few puzzles. Hexcells Plus is the expansion and has more content than the base game.

In my opinion, instead of releasing more games with just a few levels each they should give the game a workshop for players to develop their own content. That would keep the players happy because they have an infinite amount of possible puzzles and the developers wouldn't need to stress about their "skinny" game. Maybe they can even make some sort of random puzzle generator that creates a random puzzle for you based on small, medium, or large puzzle size.