I'm not particularly clear on how this is supposed to work, but it seems like a Large-Sized Light Crossbow is better in most respects.
Using an inappropriately sized weapon applies a -2 penalty to hit, and raises the weapon by one 'category' (light->one-handed->two-handed->unusable). So, to use your example, a Large Light Crossbow, while it would do the same damage as a Heavy Crossbow, would have a penalty to hit and a shorter range. There's no good reason to do it.
Further, depending on your GMs interpretation of the RAW, it can be argued that an oversized crossbow requires two hands to shoot, and cannot be reloaded, as those tasks require one hand and two hands respectively with a properly-sized weapon.