You cannot get bigger battery (with more mAh) without increasing physical size or changing the type from LithiumIon-polymer (LiPo). N900 supports only LiPo-batteries.

So the choices are only Mugen 2400 mAh (increases thickness 7 mm) or normal size 1320 mAh. Anyone who claims to have more than ~ 1350 mAh LiPo in the standard N900 BL-5J battery case is probably lying or breaking the laws of physics and chemistry.