I haven't fought any of the big bosses yet but the mini boss battles have not been as difficult as I had anticipated. Is this just me or have other also experienced this?

In general a well build party will become very powerful by act 5, being able to deal with any encounter the game can throw at you in 1-2 rounds. There are 3-4 fights that can make you struggle in act 5 though. But this is very very different from person to person. Depending on game difficulty, party composition and character builds experience can vary drastically.

Try Number of enemies increased, additional enemy behaviors on under difficulty, unless there is an achievement tied to the difficulty you are using. I always use custom so I don't know. Failing that enemy stat adjustment