Larger Ally … allied to whom? And more importantly how would we define "larger"?
Presuming the question is in regards to Romania AFTER it switched sides in late August of 1944.
So, some metrics:
Manpower applied to fighting the Axis, August 1944 to May 1945:
Romania: 538,000
Manpower in Polish forces at their height:
Poland (in the West): 228,000
Poland (in the East): ~200,000
Admittedly these numbers are not perfect, in part because while I have expressed a period of time on the Romanian number, I have only a moment in time on the Polish numbers. I have not found a reliable source for how many served over an 8 month (or even many years) period for the Poles.
If we limit our conversation to the last months of the war, I think Romania fielded a larger force (in terms of manpower). I think the Polish forces were equipped better, certainly in the west but even in the east. The Poles received reasonably current equipment from their sponsors, in numbers that matched the then-current force structures of their sponsors. The Romanians had no such luck, having to fight mostly with what they had on hand at the switch (and they were certainly not well provided before the switch).
Still the Romanians were immediately effective, taking some 60,000 Axis prisoners in the first week or two after they switched sides. So while I expect the Poles were more effective over a period of time, at the time immediately after the switch Romania was very effective.
At least that's my take.
-Mark
(aka: Mk 1)