I think that the contracts reduce in proportion to the production of the producing corporation. When I create a corporation the hiring is 100% by default. I create a load of contracts to feed the new corporation. Then there is typically a phase where I adjust some numbers and t hiring level drops. The production level drops in line with the hiring level and the contracts all drop by a similar amount. The annoying thing is that the contracts never recover despite the production level recovering.
I dont htink there is a way of preventing this. When the corporation reaches 100% hiring again I will redo the contracts if they drop below 75%, other wise I dont worry about it.
I hope that helps.