You could also argue its down to resources and production time as smaller arms would require less resources and time to manufacture which in a time of war when so much is lost is probably important.
Also the more complicated the weapon the more that can go wrong with it so it would be less reliable and require more maintenance.
It may also be down to the devs wanting to clearly differentiate the covenant and human weapons although that said it is likely just because most like the rata tatata rather than pew pew lasers =D
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