I was just thinking about Colossus and remembered that he got bigger when converting to his metal form. I looked into it and he does gain about a foot as well as DOUBLING his weight (at least). I was just wondering where he gets all this extra mass?

At first a layman might think "he's metal now so of course he must be heavier", but if you take a moment and think about it, he should weight the same. He transforms his body to metal (osmium), meaning the cells he already has are merely converting. Unless some other process of osmosis or magic and the like is taking place that we don't know about, his mass should remain the same and therefore his weight. As metal is more dense than organic material him being taller would mean he's essentially hollowing out his insides as well to increase his size in area.

Is there anything canon that explains why he gains weight or is this just something overlooked?

I think there is an explanation. I think the conversion process does make his body more dense, and this may be done by expanding and hardening his cells. If this was magic I'd expect his mass to stay the same, but he is going from a body of flesh and blood to one of a metallic-type element - which in general is heavier, and larger compared to the typical body weight of a human body. Especially given that his skeletal system would need to get larger to compensate for the increased weight.

I always took the opposite track than you did in your last sentence. Since metal is more dense than organic material, him being taller would mean that his internal organs were also gaining extra density, which overall increased his size and weight. And we've seen that he weights a lot more, part of this increased weight becoming strength, so this doesn't seem as mysterious to me. I mean, weight would increase, while muscles would appear larger if you used creatine since it helped store more water so I'm sure converting relatively soft organic material to a denser material would result in greater size and strength.

There was a book you might find interesting: "Science of the X-Men"

But you have to think about his mass. Matter is not made from nothing. He has a certain number of atoms forming his organic flesh and when he changes his cells composed of those atoms to metal he still has the same number of atoms. This means that to make the denser metal out of flesh he'd have to essentially compact it. So you could say 10 cells group to make 1 molecule of metal.

Unless he gains atoms from somewhere like osmosis or magic like I said his mass would be the same.

Writers have stated on panel that the additional mass comes from another, perhaps extra-dimensional force so perhaps your question is moot. I don't have my official handbook, but the Marvel Directory, the Wikipedia page, and the ComicVine profile all repeat this same line.

Take it as you will but the extra-dimensional source explanation definitely makes sense in light of his siblings' powers - Mikhail could manipulate subatomic matter and energy fields, while Illyana is an extradimensional time/space traveler. So there is precedent for his family having ability to transcend physical rules.

I never read about the other dimension but that does explain it. So the mass he gains is from new atoms from a different dimension, and it seems that his actual powers are psionic and not actually some for of shape-shifting like Mystique.

"While in the armored state, Colossus possesses the same degree of mobility that he does in his normal form. The conversion from flesh and bone to organic steel is accomplished by a psionic whole-brain interface with an ionic form of osmium, an extremely dense metal, located in another dimension. In willing the act of transformation, Colossus actually exchanges osmium atoms for his carbon atoms. The psionic interface with the other dimension re-creates all of Colossus's body in functionally similar organic ionic-osmium materials. The process by which Colossus gains additional mass from an unknown, perhaps extra-dimensional, force remains unknown. Colossus cannot become partially or selectively armored; his body is either entirely converted or not."

http://www.comicvine.com/colossus/4005-1460/

So while it is not explicitly stated how he gains mass, it wouldn't be unreasonable to explain it with the interaction between dimensions. Perhaps mass is calculated differently in the other dimension or he's not exchanging his carbon atoms for osmium atoms at a 1:1 ratio. Either way thanks for pointing that out. I knew there had to be some other force at play unless they just happened to overlook that.