Monarch status gives +10% income(along with other buffs
http://wiki.utopia-game.com/index.php?title=Politics). The bigger the prov, the more peasants they have, the more that 10% gives. For this reason, you will see that most of them are also dwarves(and although you cannot see, they are also merchant), further increasing the total income.
However, this does raise one question for me, which is how come KD's that are aiming for nw/land and thus are unlikely to actually enter war are as concerned with making cows? I think I understand how they are used strategically within war, but what is their purpose oow? Put differently, why have one cow/bank instead of just having everyone larger, or like 2-3 larger provs(ala dreams last age). Especially when the whole KD is already dwarf/merchant(as opposed to say undead/rogue). Is it largely deterrent, and for hostile wars? Even for someone like Pulse this age I would get it if they were running some elaborate scheme with their orc/merchants constantly trading particularly cheep soldiers for gc, but of course they don't have a cow.