Sorry, no can do: lose 4 pounds off your waistline and you'll be exactly Medium.
OK, seriously, we just bought one for my better half and with her being more "medium" than "large" and after LP salesguy saying "Cressis tend to run biggish", she still ended up sending it back for exchange. So my advice: take a very close look at the shop's return/exchange policy.
Sorry - do you mean the medium turned out to be too small?