Backup sump pump discharge too big
I fail to see a problem here. Increasing the discharge pipe diameter does not increase the load on the pump. Unless you have a pump manual that specifically forbids this (which seems very unlikely) relax and stop worrying about it - or worry about the battery running out, which is far more likely to be a problem in any extended power outage. 

If you have such an unlikely manual, replace the pipe that offends you/it.