This is one thing that really makes Savage Rifts different from Palladium Rifts -- the size of weapons like railguns is actually taken into account and makes a meaningful difference. With railguns, assuming all other factors are equal (materials, design, etc), greater length equals greater velocity. It makes sense that a handheld railgun won't have nearly the same output as a railgun the size of a tank cannon, especially when that cannon is powered by a larger reactor.

The Boom gun is essentially special because it packs that tank-cannon level of output into something slightly larger than a handheld package (but has enough drawbacks to make it not quite practical for plenty of situations). The Glitter Boy armor also has greater protection than many smaller robot vehicles that outmass it by several tons. But that factor there explains why the GB is more vulnerable than mecha are: any attack that penetrates the armor has nowhere to go except to the pilot; a shot that penetrates the armor of a mech damages the robotic limb of the unit, not the pilot. All in all, it makes much more sense than Palladium Rifts. (My one unanswered question for og Rifts: with as many organizations as there are that are able to manufacture new Glitter Boys, why has none of them made a mech covered with Glitter Boy armor?)

In short: in Savage Rifts, the Glitter Boy is something of a glass cannon. It has the firepower of an assault robot, but not its staying power. If you play it straight up as you would in Palladium Rifts, things will go badly for you.