Welcome. Thank you for choosing Just Answer.
the earlier models (1995 and earlier) use a smaller unit.
are you sure you ordered the correct kit for your bike???
check the part number and get back to me. I will help you figure it out.
if you purchased the kit from Biketronics and it is for a Harley Electra Glide and the radio is smaller than the hole, you must have received a kit for a 1995 or earlier model.
all I can suggest is to contact Biketronics and see if you can work out a swap for the correct unit.