James, compact pistols are physically smaller and have 3.6 inch or shorter barrels and mid size pistols usually have longer, up to 4.2 inch barrels. 4.6 inch or longer barrel pistols are considered full size. Photo example here : thetruthaboutguns.com/gun-review-beretta-px4-storm-full-size-compact-and-sub-compact-9mm/

there is a lot of crossover in pistol sizes I think of mid size as being a small (shorter barrel) full size pistol with a full 4 finger grip and a compact as being a slightly smaller 3 finger grip or 4 with a pinky extension

Typically, compact pistols are smaller than their mid-sized version. Keep in mind that some compact firearms may be larger than some mid-sized firearms. Typically, they are smaller, but it also depends on the firearm make and model. Usually, compact versions hold less rounds and compared to mid-sized and large size firearms. It was my pleasure to assist you.

Compact is very small mid sized is just a bit bigger