Not sure where that is true, since all games where BoS are shown as an army with military agenda are either non-canon or close to be non-canon. BoS are a secretive small organisation, that has high tech weaponry and armor to facilitate small squads with high survivability. This thing with battleships and giant robots are just fan-fic level of canon.
West Coast Brotherhood, yes. Those guys were beaten into submission by the NCR and, by the time of New Vegas, barely existed at all anymore. Their leadership is supposedly still at Lost Hills, but who knows.
East Coast Brotherhood, not so much. They expanded a lot, because it never had any real opposition. There were no other factions vying for control of the Capital Wasteland, except when the Enclave ran there after the destruction of Navarro. Due to Lyons' unique style of governance, they probably had a lot of new recruits and sympathizers.
Maxson's a bit more old-fashioned in his approach to leading the Brotherhood, which makes sense considering his lineage.
And in a future where we have Power Armor, stealth suits, super mutants and ghouls, having a giant robot doesn't seem too far-fetched.
Only thing that really doesn't make sense is why Maxson would send so many of the Brotherhood to the Commonwealth, including himself. "He's a warrior", sure, but you'd think the supreme leader would stay behind at the Citadel. Who's even in charge in the Capital Wasteland now? It's just odd.
