But they are still quite tiny, aren't they?
Not really - fighting near a gas giant can make it take up most of your screen (1000 pixel diameter maximum). Basically, about as big as they can get without the textures looking pixelated.
Also, it's now OK for them to be relatively small compared to ships because, being part of the background, they're conceptually pretty far away. Whereas if they're an object in the foreground as they are in missions now, they are rather apparently too small.
I just assumes the disproportional sizes is just the in-game strategic map used by people to make it easier for people to see stuff.
Yeah, that sounds like a really good way to look at it.
A realistic scale is one of those things that, while in theory nice, just isn't going to happen for practical reasons.