Why doesn't the presynaptic membrane keep growing? When the synaptic vesicles are binding to them, releasing acetylchloine, their phospholipid bilayer membrane is surely just adding onto what's already there in the membrane? At first i thought they would invaginate back into the pre-synatic membrane as in synapses, however the neurotransmitter is broken down by an enzyme.

The membrane of cells is basically a phospholipid bilayer. Things add to and bud off from it all the time so overall there is no effect on how big it is.