What would he cut? If you look at his website and the priorities section, Defence doesn't appear but Peace does. I suspect he would cut everything back to a gendarmerie type function whilst bolstering the likes of DFID and various other departments on the tree-hugging side of politics. Less talk quietly but carry a big stick, more stay silent and give your enemy your stick.
Corbyn is positively dangerous for this country on so many levels. Yes, he's a bit of a joke at the outset, and there are many that will enjoy seeing Labour become unelectable for years, but this is a sort sighted sentiment. With the backing of so many of the Unions, Corbyn doesn't need to become PM to wield power and influence. He can get the Unions to carry his big stick for him. He can shout from the Opposition benches with impunity, knowing he will never have to stand on the other side of the House; the Unions will do his dirty work for him and we will enter into a period of anarco-syndicalism where the Government will spend all it's time trying to counter Union disruption and action, spurred on by Corbyn's unique brand of 6th Form socialism. They won't have the time or capacity to do anything vaguely constructive or productive other than countering Union action. Further, Corbyn will represent only a small section of the Labour Party leaving large elements on the centre-left effectively unrepresented, clearly not a good state for a democracy to be in regardless of your political persuasion.
Laugh all we like now, but things could become very complicated very quickly if Corbyn gets in. And if he does, don't count on your new build AH-64s, F-35s or carriers any time soon.