Firstly, the 'type' of a ship is not it's 'designation'. Just because the Battle Barge is a type GC doesn't mean it's not a battleship!
Admiral_d_Artagnan and all,
The 'why' make a Battle Barge smaller seems to be quite subjective.
Easier to field: My 'main' reason for the change is to make it easier* to field in smaller games and keep it balanced. (*Having the first BB available without limitation is ridiculous! So assuming the current rules were per full 1000pts like a Void Stalker.)
This also makes it easier to make a fleet list and adds variety as you can field more BB's breaking up the wall of SCs.
Can't be squadroned with armour 5+ BB's: This is minor, and wouldn't really have been noticed by those that aren't obsessive but it is a game effect that doesn't occur in the Chaos and IN list (well you can get an armour prow for IN), it would need factoring in for cost. As the BB is the mainstay the 5+ armour choices would have to cost a little bit more.
Better Turning: Given that SC's have a 90* turn having the BB have a 45* turn with the same minimum move seems reasonable. Where as 45* after 15cm is verging on a extreme difference. Being able to turn like this will really help the BB keep with the Fleet rather than being it's own detachment, or 'drifting defence' in comparison.
Better Speed: Now this one doesn't need the 'GC' element. The GC status just makes it fit the speed trend given its armour rating.
Reduction of hits matching the theme: This also doesn't need the 'GC' status. Having less hits makes it a more natural progression of size from the SC rather than 'doubling up'.
Attack Rating: This is just a matter of opinion, as attack rating represents so many logistical factors. It being faster really helps for this, but that's just one opinion...
Horizon,
1 TH on the SC: That would be almost unuseable. They would have to be used in squadrons and that doesn't feel right. (or TH's get a 4+ sv vs turrets!
)
Sigoroth,
Model size: This isn't too crucial, model size can vary for hits, even though the Battle Barge is smaller than most other BB models.
zaxqua,
'BL says so' is a crazy point of view, but in any case perhaps the Space Marines on board make it one of the most powerful battleships in the galaxy. But to be fair there aren't really that many Battleship classes anyway!
Is it really the GC 'name' that bugs people? How about a 10Hit BB that can turn after 10cm and can CTNH?
Cheers,
RayB HA