Sometimes when big size pictures are posted, they get resized smaller with the option to click to make it larger.
This is great!

BUT this doesn't always happen.
Is there an explanation for this?

have you set up picture resizing on your forum?

acp - general - messages and emails - configuration - Images resize

I think so. Are there further settings that could be doing this?

well can you check? it's the main way to do it.

Are the ones that are not being resizes the size or smaller than the sizes you have set?  If they are, they will not be resized and have the links to enlarge or full size.