I bought Falliut on here when interplay still sold it, thus I have the classic version. Is there any reason to buy the bethsoft release of the first fallout game?

No. Just a change of ownership.

The classic version is a smaller download (502mb vs 687mb in 2 parts) and includes a bunch of extras (EG: Wallpapers, artwork and avatars)
Not sure what (if anything) is included in the bigger file size for the non classic version

I was perplexed too. I saw Fallout Tactics on the store page and it said I didn't own it, so I checked and my copy is indeed labelled classic now.
It is a bit odd how they are considered to be different games, when apparently there is barely any change.