Due to certain previous conformity and legacy issues the Thumbnails object had a structure of small, medium, large and fullscreen fields. Ideally there would have been standard dimensions for what constituted these sizes but these were never formally defined. The current implementation has small(20x20), med(30x30), large(50x50), fullscreen(320x280) which were meant to be placeholder dimensions that could be easily changed later. There was never enforcement as to whether a particular Thumbnail was actually the same or had close to the ideal dimensions of a standard size. E.g. it was possible that someone could place a 70x70 thumbnail into the small field and a 10x10 icon into the fullscreen field.
I believe the whole concept of thumbnails and what use cases they solve needs to be re-evaluated. The concept of small, medium and large introduces extra complexity that is not really needed. A single thumbnail image can be scaled up or down in size as necessary.