Something to also keep in mind is that giant enemies don't work that well in Smash due to things like Blast Zones.
This is actually notable in Smash Remix, as Giant DK is very buggy in general, actually being lighter than other variants, and having a lot of gameplay issues. This could be part of why Giga Bowser isn't playable as well. And we also saw bugs with his size, sometimes being sucked into the blast zone way too easily while jumping(this is ignoring that he's given boss traits that would be removed if he was playable, like being immune to throws, of course).
So it makes sense everyone is reduced to a degree(and we do see how jank the Super Mushrooms can be or simply using the mode options to enlarge a character). It's not meant to be a normal size nor is it meant to be balanced.
So yes, reducing their size is actually necessary for balancing reasons. They're funny things, but do not work well anyway. Also, keep in mind other games are different kind of engines and may incorporate different programming that takes care of issues in a way Smash isn't designed for. It's not that they're bad ideas at all, it's just that they don't translate well. Size does matter.
Anyway, part of Kraid's issue is a pretty severe lack of unique animations. His boss fight is extremely limited, so making a feasible moveset is a lot harder due to a massive lack of material. Mother Brain suffers in a similar way, though putting her in some kind of mech while as the Brain in a Jar version does alleviate that. Her monstrous form also can move just fine, though isn't, as I said, heavily utilized to have a much cleaner moveset concept. As for Captain N, it's a fun memorable thing and all, and despite Nintendo owning it, it also was not often faithful to many characters. So outright using designs from them isn't really the most logical thing to do. Hell, the CD-i Zelda games had sometimes more faithful designs(despite the jankiness of them, you can clearly tell who they are). That, and keep in mind they're based upon much older designs too, so Ridley looks way more like his older artwork than his more updated designs(which Smash works with instead).
Another factor is that Captain N is long forgotten, and outside of DiC having the exclusive rights to release the show, it's pretty much ignored by Nintendo too. Most likely due to the hefty amount of issues it has(Samus being actually worse than her Other M incarnation personality-wise. Yes, it's crazy. While she's a bit off in Other M, these aren't actually that unbelievable. Having PTSD from seeing Ridley makes a ton of sense. He caused tons of trauma for her. Her reminiscing about the Baby Metroid is absolutely a normal thing anyone would do. It was very special to her. Adam was her commanding Officer. Her following orders isn't weird either, though him being a bleh person is more of the issue on its own. His decisions are pretty damn stupid at times, so it's not that Samus shouldn't respect him so much as he's given a bad personality to justify it. It's still awful writing in that regard, but not as damning as being exaggerated as). As for the Comic, Samus has never been a person who suddenly has romance on the mind. Her suddenly fighting with Lana over Kevin's affection is far more massively out of character than what Other M did. She's still a trained person in military affairs, and she's still a regular sentient being with feelings and trauma. Other M just portrayed it badly, yeah. Hopefully they learn to do a bit of a better job at it, but still make it clear she's a person, not an empty shell. That's the only reason I like that she was given more emotion outside of the manga. It made sense.
