An OS update is executed using the software code (and the other setup) already in the device. The new update cannot help the installation of itself! The code of the new update will be in effect only after the device restart which is done in the end of the installation.
So, if a phone has 4.0.1.45 in it then the update to 4.0.1.48 is done by running the instructions contained in the code of 4.0.1.45. Hence, if there is a problem in installing 4.0.1.48, that problem is most probably caused by something in the phone now - the changes in 4.0.1.48 do not affect at that point yet.
The download size from 4.0.1.45 to 4.0.1.48 is around 30 MB. Only the modified code packages are replaced.