When laying at SP with Betfair you are laying to a fixed liablity. For example when laying fav £5 at 1.2 you are laying to a liability of £1. (ie 5 times 0.2) If actual SP odds are higher than 1.2 then your stake is reduced so that your liability remains at £1.
If actual SP odds are 1.2 then your stake is your liability of £1 divided by 0.2, = £5 stake
But if actual SP odds end up as being 2.2 then your stake is £1 divided by 1.2, ie 0.83. This is correct behaviour in the way that Betfair works with SP lay bets.