That depends on how big Ridley is compared to Samus.
That depends on the game, really. In some (particularly the 3D ones) he's been portrayed as absolutely massive, but in others he's comparably small.
There's also the issue that there's like 4 different Ridley versions too.
Between her role in 3D World (getting Mario's moves from Galaxy) and Smash (using a lot of moves based on elements of the Galaxy series), I think that they're just making her the representative of the Mario Galaxy series from here on out. It doesn't hurt that she's gotten really warm fan reception and has cool design and backstory, though. But I think that's about all there is to it.
I guess. It just feels like more is going on than just representing the 2 Galaxy games. I mean, we didn't get somebody representing Mario 64, for instance. Heck, most of the unique enemies in that game never made it into any other game (I'm trying to think of some that did, but ... none really come up).
Or somebody for Mario Sunshine. Bowser Jr. is the closest, I guess, but since Sunshine, he's been the antagonist for other series as well (in particular the NSMB games), and he didn't even use the paintbrush there.
Wario and Daisy also could barely be called to represent the Land games. There's been absolutely no references to those 2 games in any other Mario games they appeared in, except for a couple of tracks from the first SML. But we've never seen Tatanga again, or visited Sarasaland, or Mario's Castle, or heard of the 6 golden coins again.
I dunno, it just seems weird to me. I'm pretty sure they're pushing her a bit to add her to the main lineup. For some reason. I dunno.
Wouldn't surprise me. Has she appeared in Mario Party yet? Because I can see her appearing in Mario Party 10 (if the WiiU ever gets one). Possibly her own game as well like the other mainliners (except Toad) have got. Also the possibility of Rosalina's costume appearing in third party games on Wii U (what few there will be).
She's not appeared as playable in Mario Party yet, but I dunno about the 3DS Mario Party game. I only heard that she was going to get her own board dedicated to the Galaxy series and her. Also that she only wasn't in the 3DS Mario Tennis because of time restraints, which is why they added in Luma (that was said by the producer or lead designer of that game I think, in an interview).
Also Toad has gotten an own game, it's called Wario's Woods. Really late in the NES' lifespan, though.