The Swedes lost 2100 men. About 6000 Imperial troops were captured on the battlefield, and a further 3000 surrendered in Leipzig the next day. It was not so much that some of them changed sides, but that they were forced to change sides. This made up -- more than made up -- for the Swedish losses. Impressment and recruitment of POWs was common in the Thirty Years War, varying from less-than-voluntary impressment to changing sides for the better paymaster. There were other battles where the winner's army grew in size, from either cause.

A point of significance is that often the pay of soldiers was much in arrears, and soldiers who switched sides would forfeit their back-pay (unless their new paymaster was willing to pay them an equivalent bonus for changing sides, as some did). This was enough the case that it makes me wonder if tardy payment was a deliberate strategy to discourage desertion and changing sides (both of which would forfeit back-pay).

For an excellent history of the Thirty Years War, including both the military maneuvering and the politics, see Peter H. Wilson, The Thirty Years War, Belknap Press, 2009. History as she should be written, and I'd have finished it a month ago except for it being a thousand pages of juicy goodness coinciding with grant-writing season.