You'll only be able to get the XXL B.O.B. when you get C4 which is obtained through progressing in the story.
*Update, thanks to Razorpriest for reminding me, there are two locations for the XXL B.O.B one is story related you can pick up after completing the arena at St. George's Church.
The video I used is for the other XXL B.O.B that can be accessed earlier on in the game.
Here's a video credit to Harryninetyfourhttps://m.youtube.com/watch?v=bM7m6ChEz3Y