Sniper rifles can be very light for infantrymen to handle these days. The SVU-AS weighs at around 4kg because it's a bullpup rifle. LMGs are still heavy in comparison, although they have become lighter themselves over the years, and the mobility issue makes sense. I use LMGs myself and have no qualms with the lack of mobility.
I wouldn't mind seeing specific mobility percentages for each weapon within a category of weapons because drum fed LMGs could well be a lot lighter than belt fed ones but still suffer from the mobility of heavier LMGs.