I am not sure, but I think, that orthomosaic file is internaly in uncompressed format, like simple bitmap 8bits/channel.
For example 18Mpix jpeg from camera has around 10MB in highest JPEG quality, it's bitmap equivalent is 51MB... 5 times more. If you make some overlap when taking photos, then 3.2 times bigger orthomosaic seems to be correct and minimal size.