They're tiny in their own games, but they're proportionally "regular-sized," so it's not at all jarring seeing them measure up to the other characters in Smash Bros. On the other hand, we've seen Ridley tower over Samus in the Metroid games, so to suddenly make him small would be confusing.
There are Pikmin in Mario Golf Toadstool Tour, and they're as small as they appear to be in the Pikmin games in regards to the world around them.
Now, you can argue that it's just an Easter Egg, and can be dismissed, but there's still the fact that Pikmin appearing as large as the other characters in Brawl just looks "inaccurate". Because the whole point of the Pikmin games is using dozens of them to defeat significantly larger opponents, seeing one or two large Pikmin clinging to Brawl characters seems no less out of place than seeing Ridley with the same proportions as Charizard.
Also, let's not forget that Charizard is supposed to be huge as well. The best example of this is in Pokemon Snap. Ironically, he's roughly the same size as Ridley in the Metroid games. Squirtle, Ivysaur, and Charizard aren't to scale at all.
But in the end, as others here have pointed out, are people so concerned about characters being to scale, that they'd rather have some of them absent entirely rather than seeing them bigger/smaller than they should be? There's no logical explanation given as to why all these characters are fighting in the first place, so I really don't see why people are arguing that certain characters shouldn't be included because changing their size "wouldn't make sense". How is it that Solid Snake fighting Link on a Mario Kart track is perfectly acceptable, but seeing Ridley a bit smaller than usual is too much of a stretch?
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