Here, this explains that basically the down-scaled rifles were made because they were cheaper, and were easier to handle:

"Initially, the weapon was a bulky, industrial plasma caster, manufactured by Winchester Arms; while it performed admirably, it was expensive and required specialized training to use. However, later developments for the United States Army would lead to a more compact design that was cheaper to produce and easier to train for, perfectly suited for urban warfare."

In other words, the energy rifles and pistols serve the same purpose: to be smaller, cheaper, and easier to handle. Which to me is unacceptable. Max efficacy all the way. I want all the damage, no budget cuts.