An 18MP sensor can record finer details than an 8MP sensor (of similar size), therefore the 18MP image can be enlarged more whilst still retaining more detail than the 8MP image. So, "enlarging" to a fixed size is a flawed normalisation method. Besides, DoF is a characteristic of lenses, not sensors ... so again normalising different sensor sizes through enlargement to a fixed size produces flawed results.
There is a great deal of misunderstanding in that paragraph. Too much for me to feel like trying to help you understand. Suffice it to state that you're conflating a great many variables with reckless abandon.
I'll just leave the matter (for now) by noting that this is all stuff covered in elementary photography (and physics / optics) textbooks and suggest that you take a trip to your local library (or bookstore) to get up to speed.