What's Optimal About Optimal?

Without getting much into the math aspect of this in the replies, I want to ask a simple and hopefully not that obvious question. I'm aware that there's an optimal pinhole diameter based on the focal length. My question is, what about this optimal pinhole diameter is optimal? I realize that the further away the image plane becomes from the focal plane, the dimmer the project image becomes (if the pinhole diameter was fixed). Therefore a larger pinhole should be used as the focal length increases so exposure time aren't ridiculous. I also know that sharpness (resolution?) decreases as the pinhole increases in diameter. I'm asking this because I just built a pinhole camera with a focal length of 190mm and based on a couple websites I should have a pinhole of about .58-.62mm. I feel like that's large compared to my previous cameras. With my previous pinhole cameras (at shorter focal lengths) I've been using home made pinholes of around .30mm and have gotten pretty sharp images with it.

Anyway, my question boils down to what is optimal about optimal and what would be the problem with using a smaller pinhole (to preserve sharpness) in a longer focal length camera? Any replies greatly appreciated -thank you.