Xtol has a far higher capacity because it doesn't contain Metol. the Bromide build up is the limiting factor with D76/ID-11. Ilford introduced Autophen a PQ variant of ID-11 as a commercial photo-finishing developer and it had a very much higher capacity.
Ilford suggest up to 10 films per litre (35mm 36ex or 120) per litre of unreplenished ID-11. If you use Xtol then replenishment is a far better option and it's simple even on a relatively small scale.
Many thanks, as far as I'm concerned that explains it. I might take it up to 10 for D76 (or even 11, if I'm feeling really wild).