I suppose they didn’t want to bind it yet again to a unit and go with a name they could use in many countries. 113g burger would’ve probably worked. But increasing it to 125g or 1/8 kg would probably been better.
In Germany McD at first literally translated its quarter pounder as Viertelpfunder, but then switched to Hamburger Royal. They might have realized that that was actually 125g and not the intended 113g. Pound isn’t a context-free or precise unit.