What does the chapter 3 quote, "Give 'em all the same grub...done in a day," mean?

The people who make declarations of war are never the people who have to do the dirty work of fighting it.  Kat's ironic statement,

"Give 'em all the same grub and all the same pay and the war would be over and done in a day"

refers to this reality.  He is in essence saying that if the people who started wars - the politicians and world leaders - were forced to experience the hardships the front-line men endure ("the same grub and the same pay"), they would think twice before committing themselves and their countries to this absurdity.  Kropp expands on this idea, proposing that

"a declaration of war should be a kind of popular festival with entrance-tickets and bands, like a bull fight.  Then in the arena the ministers and generals of the two countries, dressed in bathing-drawers and armed with clubs, can have it out among themselves.  Whoever survives, his country wins".

The ridiculousness of modern warfare is only too clear to the fighting men.  All agree that an arrangement such as that suggested by Kropp would be "much simpler and more just than (the present) arrangement, where the wrong people do the fighting" (Chapter 3).