Text and database entries are, indeed, Vastly lighter on resources (compute and bandwidth). Bandwidth cost for text is practically free. Managing voting is surely not easy though; every single vote is an additional database entry which means that they cost money to store. Then the vote sorting algorithm has to run pretty frequently. Comments themselves are probably nothing compared to the votes.

But managing voting and sorting is a one-time-cost. Reddit doesn’t let you vote on old things which means that they can completely discard the votes and keep the totals and never resort ever again. They have no real ongoing costs there.