When are Rosalina and Mewtwo ever explicitly stated to be heavyweight individuals? Yeah, they don't exactly move around much and Mewtwo's Pokédex entry proves that he's not exactly a pushover or anything, but why on earth would that translate to "martial tank"?
And yeah, of course Ike was planned to be heavy from the beginning. BECAUSE THAT'S HOW GAME BALANCE WORKS. You don't just take a character who's super fast, super strong, super agile and super durable in their source material and have them fight the exact same way in a game that's supposed to have an essential tenet of balance to it: that's just awful design, and it ensures that there's no point in even playing the game because who could possibly beat the unbreakable killing machine?
There's also the fact that Ike is designed as a contrast to Marth: the latter is a graceful and lithe swordfighter whose style resembles fencing, a sport traditionally favoured by nobility and those of well-heeled upbringing; contrast that with Ike, a mercenary who doesn't have a trace of royal blood in him and who learned to fight from his father, a homespun hero that made a living off of getting his hands dirty and cracking skulls for coin. The two styles are naturally going to be very different, and this is reflected in how they fight. The differences are further hewn in Smash, wherein Marth is the fast, athletic swordfighter and Ike is the brash, brutal warrior, the latter an archetype that had never appeared in Smash before Brawl.
tl;dr - Why is Ike a heavyweight? Because it works, and that's that.