1997 Toyota Corolla IDLE Unstable

Hi there.

I have a Toyota corolla 1997 model. I have notice the idle is unstable, the mechanic says that there is vacuum leak on the brake booster and so we replace the brake booster to a new one but not the identical size its a little larger but it is still toyota. After replacing, at start the idle gets worst it reaches to about 2200rpm for several minutes and then get back to 800rpm when the engine heats up.

whats wrong with my car engine? Thank you in advance