Question about Image Scale

Hi,
I've been observing for a couple of years now, Mostly with small scopes of less than 5" aperture. Every once in awhile I hear someone talk about "Image Scale", hinting that for the same Magnification (say 80x) an object will appear larger in a larger scope. I always assumed that the only thing that affected image size was magnification, but get the feeling I'm missing a core concept.

Can someone elaborate a bit on this for me, and provide the Mathematical formula, if one is available.

Thanks in advance,
Steve

Image scale refers to an image formed in the focal plane, not the one we see through the eyepiece. Image formed by the objective is real, and formulas that Jim gave are for it. Image seen through an eyepiece is magnified by both objective (Mo=fo/250) and eyepiece (Me=250/fe), resulting in the familiar total magnification formula M=MoxMe=fo/fe.
In other words, even if two instruments have identical total magnification of 80x (or any other), it is quite possible that one of them has larger image scale. As Jim points out, it would be one with longer f.l.