Its not so much cost/size of the chargers, but rather the "cleanliness" of the power delivered to the charger. If it was related to the size of the chargers, we wouldn't see as many gfx's as we do. When chargers come with internal power supplies, they are usually of poor quality and have a low amperage rating. These units sometimes can leak some ac current which is bad for the batteries.
1989 Mazda Rx7
greddy t04b, megasquirt ecu, PlX wideband, 12x28x3 fmic, 50mm hks clone wg, ssqv bov, Safc, ss oil lines, walbro fuel pump, tubo intake manifolds, ssac exhaust manifold, 3" dp, 550cc primary/720cc secondary injectors, etc.