I know some of what it takes to make an army move, let us here deal with an ancient one; the Roman. When the Romans started a campaign they would gather ALL sorts of provisions in abundance and advance. The numbers I know of speaks of six millions bushels of wheat gathered for the campaign of Constan(something) against his nephew Julian (later "The Apostate"). There is also the famous incident of the same Julian's campaign against the Allemans(? writing from memory) where grooms were working with a stack of haybales that collapsed and killed 50!! men (and the haybales of that day were not the 10kg ones we know today, they were significantly smaller bundles). The same campaign saw hundreds of wagons following the troops with arrows, horseshoes etc. and I seem to remember that Julian only had 13.500 men on that campaign (somebody with the numbers at hand correct me if I am grossly off).
Now, admittedly, the Romans were the "Americans" of that day, defeating opponents by logistics, and many later campaigns were much more disorganised and ill-supplied (many germanic and medieval armies starved on their campaigns), but this speaks somewhat of what it takes to keep a large army running smoothly. I think that 1. Sauron had not yet gathered significant supplies for his offensive campaign.
2. He prudently kept a reserve for either a follow-up stroke or defence, all sensible military leaders always keeps a reserve to counter the unforeseen or exploit the critical breakthrough.
Further, the forces assaulting Gondor may have been as large or larger than the one kept in Mordor, but they were defeated piece-meal by miraculous events that Sauron could not have expected ("An army of dead? Fool, I am the only one to raise and control armies of dead- nobody else is powerful enough").
And why should he waste his own troops (that he needed for the final conquest) in bloody battles if he could make others fight for him (corsairs, Haradrim, Variags, etc.)?
NT