Okay, so I've been away from this and it's definitely high time to respond.
When scaling the icons, I've mostly wanted to roughly imitate the size of the objects as seen from the player's perspective, mostly using typical FS2 mission space distances to get the "feel" of it. Sathanas and Colossus are examples of objects that basically function as semi-interactive backgrounds to the scene, so I've made them big, just as most larger capships. Also in some cases, I've decided to make an icon slightly bigger if the ship serves some important function - hence why support ship icon is larger, as you are looking for it pretty often and I suppose it should stand out a bit.
If you've got suggestions for roughly which icons should be rescaled, please do tell. Do you think fighters need shrinking or it's more about large ships?
Also, regarding destroyer icons - the same case is both for Shivan and Vasudan ones. I've just decided that it would be better to distinguish between the races instead of being precise with ship classes. After all, there's no icons for the Demon and Typhon, at least ones that I know of, and breaking immersion for FS1 doesn't get too much attention from me as I kinda expect the majority of the players to have played FS2 before or not paying large attention to what exactly is on the radar icons. Besides, FSPort currently uses its own set of icons from what I know and that probably won't change until it gets updated to cooperate with new MVPS packaging.
Of course, if anyone would be willing to make some of the missing icons, that would be really appreciated, and quickly integrated. Remember, I've just snatched the readily available icons and configured them for MVPS.
I actually remember going through most of resolutions up to 1080p and checking if the 2D radar doesn't get whacky with the icons. Results - it seems like the radar gauge in MVPS-4.0.0 (FSO 19.0.0-RC1) always takes up the same amount of pixels, even if at 1024x768 the HUD looks gigantic and slightly obstructive. So the size of 2D radar icons relative to the radar size doesn't change. I didn't check that for resolutions beyond 1080p (got no bigger monitor), but I think it's rather credible to assume that majority of players will use 1080p or smaller resolution. I even remember talking about that in the Discord.
Of course if you know how this will act in bigger resolutions and how (if at all) is this behaviour fixable, I'd happily accept the info, or even someone just slapping it into the table file (for the sake of me being unable to try the results in action... and my laziness