I've been running an old version, 0.8, of bitcoin core on Windows 7 and decided to upgrade to 0.15.0.1
To be on the safe side, I removed my wallet.dat file and just let the install create a new wallet.dat.
However, my original wallet.dat is 96Kb, while the new (empty) one shows as 1408Kb.
That's a huge size increase.
What accounts for that? Is it safe to replace the new wallet.dat with my original?
It is safe to just replace the files. Make sure you run your core with the -updatewallet command to ensure your wallet file gets converted into a usable format.
The size difference is due to a few factors. One of which being that the new file is hierarchical deterministic meaning that it has to store a very large number to determine allof the addresses and private keys your wallet can have.