Heavy weapons are issue when there are more players on server on chokepoint-like maps (adlernest, erdenberg, delivery,...) because AoE weapons will just deal more damage if there are more players jumping around. Restricting HW if there are less players on server therefore won't solve this issue.
IMO proper solution would be to edit such maps to have wider pathways, but I understand that probably ain't happening. Maybe some other solution could be with slower recharge, but it would also have to be applied to grenades somehow.
ET and it's maps weren't designed to be played 20v20, it just happened to be quite fun on public 😄