I planted a couple seeds from the pictured plant that has little peppers on it and the 2 new plants (in 1 pot) have much bigger leaves and are much taller than the original plant which is a few years old. The new plants are only about 2 months old. I know nothing about plants, and I'm curious about what is going on here.

Seeds were probably cross pollinated. Or pepper is a hybrid.
😎😎😎

Done that too and learnt that SOME peppers doesn't come true from seed and revert back to one of the parent plants that they where original crossed from.

Janett is right. An entire ancestry of traits went into producing that seed.