When I first started collecting the Zimbabwean 3rd dollars I thought they were all about the same size regardless of denomination.
When I expanded my set to include more of the first dollars I started to notice that this wasn't the case with them. I was shocked the first time I held a P-1 note. Compared to the higher denominations in the series it is tiny. The shot below shows the $2 note over the $20.
Then I finally got some low denomination 3rd dollars - P-65 and P-66 - and I realized that this wasn't exclusive to the 1st dollars. The ZWR had it too, I think the picture below was the $1 note and the $100 note.
This was a really cool feature / realization for me.
I'd read years ago that some people were pushing to make the different denominations in the US different sizes - it's an access issue for the blind. The argument was that the current bills deny adequate access to the blind and that making the notes different sizes would allow the blind to tell the difference between them without help.
After reading that years ago, seeing this was just really neat.
I don't really see any difference between the sizes of anything after about $500 or $1000. I can only assume this was because they either couldn't make the notes any bigger (They are quite big), or because they chose to standardize around a size to make it easier to keep cranking out higher denomination notes.