Various arguments:
1) A cube is a difficult shape to gradually and evenly expand by adding random debris from defeated starships. If Borg ships grew gradually through assimilation, they might be assumed to be of some other shape.
2) Our heroes found out that the cube that threatened Earth in "BoBW" was of the exact same dimensions as the one they had encountered in "Q Who?". Now, they apparently felt this meant it was the same cube - meaning a) they believed in varying sizes, and b) they might have been dead wrong on both counts.
3) Indeed, in VOY, all those cubes that look like the TNG ones appear to be of the same size (although it's quite difficult to tell even when those fly in neat formation!).
4) In ENT "Regeneration", we see the Borg slowly accrue an all-new Borg vessel around a captured alien (that is, human) one. We don't know if that's how all Borg vessels start out, but all the VOY Borg ships that differed from the cube shape were small ones - so, potentially captured vessels slowly growing up to standard cube shape and size.
5) Did those smaller vessels have specific functions, or were they just "baby cubes" by assignment, too? In TNG, the smaller cube type from "I, Borg" was not behaving differently from the larger ones, but only because there was so little behaving going on in general. In VOY, the spheres did pretty much the same stuff as the big cubes, and the smaller plated cube type from "Unimatrix Zero" could simply have been immature, too.
6) Yet Seven of Nine did have special names for some of the types; the plated one was a "tactical cube", perhaps reflecting its lesser combat abilities as compared with the more powerful "strategic cubes" or something. This could support either the model where all ships start out small and irregular and grow to cubes 2.7 km on the side, playing different roles at different sizes - or the model where the Borg build all sorts of ships, and the 2.7 km cubes just happen to be the ones they generally send "abroad".
Timo Saloniemi
i was just looking around and saw a picture of a borg cube and the outside looked all covered in pipes and random metal pieces and looked as if it was still being built or it might supposed to be like that, but then i thought what if they just keep expnding on the sides for more and more room. Is this possible and or is this what they do? plz 
