Thanks a bunch! This was my first attempt at trying to add juice to the game and I'm pretty happy with the results!
The XP gain curve is exponential so your first few levels should come easy but it should slow down! The first level is a toss up as the enemy's strength is what the level modifier is based on! The first bug you noted is probably a result of an enemy with high strength >> I think I have to cap this as you can get a ton of exp for an easy fight if the enemy has low hp.
The doubling floor thing I've not noticed!!
Thanks so much for the valuable feedback and giving the game a try :)