That would be bad for the lipo - it could compromise it internally. You might "get away" with one 1mm of diff, but not 4 or 5mm. BTW, my experience with those has been that they are usually bigger (not smaller) that what they show in the specs.

Its better to mod the batt tray to allow it to fit than squash the lipo into place if you have that option or find another that is closer to the width you need.