It sounds like you’re doing everything correctly; three and a half feet is actually pretty tall for a ponytail palm that’s spent its life indoors! Indoor specimens taller than 4 feet are fairly uncommon. The plant really needs ideal conditions (bright sunlight, warm temperatures, and no restrictions on its roots and trunk) to reach taller heights, which is usually the case for ponytail palms grown in greenhouses or outdoors.
Ponytail palms are very responsive to the size of their pots, so keeping it pot-bound will slow down its overall growth. You might try planting it in a deeper and wider pot than it is currently; that could encourage it to grow larger both above and below ground.
It sounds like you’re doing everything correctly; three and a half feet is actually pretty tall for a ponytail palm that’s spent its life indoors! Indoor specimens taller than 4 feet are fairly uncommon. The plant really needs ideal conditions (bright sunlight, warm temperatures, and no restrictions on its roots and trunk) to reach taller heights, which is usually the case for ponytail palms grown in greenhouses or outdoors.
Ponytail palms are very responsive to the size of their pots, so keeping it pot-bound will slow down its overall growth. You might try planting it in a deeper and wider pot than it is currently; that could encourage it to grow larger both above and below ground.