Varying pungency with size
From my experience the size of chili peppers is not related to the pungency.
Some time ago I grew all my chilies at home and while some were hotter than others it had nothing to do with their size.

But there might be varieties where that is true.

Some kinds of chili peppers start out hot, get mild after some time and then again become hot. So if you compare ones not fully grown to others in their mild stage it might explain differences in heat.

One variety I grew a lot started green (hot), then turned a dark violet (mild) to turn red (very hot) when ripe. But peppers of different sizes at the same stage of ripeness had similar pungency.