No, there isn't. All characters are absolutely identical until level 5, and then differ only by skills and class mods. At level 10 an Assassin with Ir0n Hand would be more durable than a Gunzerker without Hard To Kill.
And hitboxes, yes.
I can not back my answer with explicit quotations, I base it on vast BL1 experience and some BL2. Also, as of now I did not find any source that contradicts this.
DLC correction: designers had to switch some things around with Krieg due to his unique playstyle.
Firstly, he has a much larger mass, otherwise enemies just knocked him back all the time. Hence, geysers propel him worse.
Secondly, Krieg has a built-in bonus to Melee damage, that increases with his character level. Important to note that this bonus "occupies" the slot of weapon blade attachment, and is replaced with blade bonus, if one is equipped. Innate bonus passes +50% by level 44 or so, so usual, non-Unique weapon blades become actually harmful for Krieg afterwards.