GRRM also addresses those points. When the army of the North is assembling at Winterfell in AGoT, they actually have to depart before the full strength has been gathered because they are stripping the countryside bare and will starve if they wait any longer. Dany's army actually does start starving whilst it is besieging Meereen in ASoS, spurring the need to seize the city (and its food stocks) as soon as possible. Keeping the city and its occupying army supplied and fed in the face of a native insurgency and outside enemies threatening the city will be an important plot strand in the next book.
By the end of AFFC, two years of warfare, pillaging and foraging has left the Riverlands in a state of famine, not helped by the arrival of winter and the realisation (by Jaime) that thousands will die in the coming months.
Paul Kearney's very large armies in Monarchies of God (although this is fair enough, as this series takes place in a post-Renaissance, pre-Napoleonic world with superior technology) also have immense supply problems, as detailed in the books.