IMHO that's wrong and illogical comparison because R5 cars have larger restrictor for such cylinder capacity (21% larger). If they would run smaller restrictor it's possible the teams would select smaller engines because for every air quantity there is an ideal engine size. Otherwise if bigger engine would be always better it would make sense to run 6 litre V12 with 30 mm restrictor. That is obviously a nonsense. You loose power at high rpm because you can't fill the larger chamber with enough air. With bigger engines everything is heavier you loose power by accelerating the inertia of the heavier parts, in larger friction areas, with more oil inside etc. There is no point to have larger engine than necessary.
Of course we have to take into account other factors which speak for selecting larger engines for R5 such as reliability (since they use stock engines), cooling (since they use stock engines) or large enough valves (since they use stock engine and can't enlarge its cylinder diameter) but all these three factors goes down with smaller restrictor.
Another factor is level of turbo boost. With R5 it's given and same for all. With Rally3 it looks like they don't run on the same given maximum level. Maybe Br21 can add somethign here.