As a rough starting point for your research, the higher the BITRATE the larger the file. What you haven’t said is what/where the File will be played on and shown monitor.
720p simply indicates one of the dimensions of the Frame and that it will be Progressive.
Being helpful here, why not try a suck n see attitude? I’ve calculated MPG for DVDs using a BitRate calculator, must be one for MP4s?
Does any of this help?
* Grazie
Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge