Okay, so the cannon was just a tad bit smaller than promised, but it should still work just fine. With Lani's colossal gut shoved into the cannon, there should hopefully be a good air seal so that she doesn't just fall out. While it's a tad patronizing, Lani knows that getting some of that treasure was all that mattered.
The pirates figured that sinking their rival ship wouldn't be easy unless they could practically split the ship in half with one shot, and rumors of a wailord-sized Lapras who roamed a nearby shore peaked their interest. With all that flab, she could easily break anything she falls onto!
That being said, it's a miracle that she didn't fall through the floor of their ship, especially when inside that big metal cannon.