The BOB XL gives you more room for lower kicking. Not a lot more room, but it was enough for me. It's hard to say how much, but here's the best way I can think of...
If you look at the standard BOB, you'll see the barrel on the bottom that goes over the shaft. You can barely see it on the BOB XL. It took a Dick's Sporting Goods salesman and I staring at their website to spot it. I went over to the standard BOB (they don't have an XL on the floor) and threw few a quick punch-roundhouse combos, paying attention to where my foot hit. My foot hit where BOB XL's thighs would've been every time.
If you have a local store with the standard BOB, try it out, noting where your foot lands. Just don't kick hard
I've also see people on YouTube wrap padding around the shaft, giving them more room to kick. And I saw a guy put a kicking shield around the base, and had it held on with bungee cords to keep it from moving. None of those are perfect solutions, but they seem to get the job done. I'll experiment when the time comes and I'm motivated.
I still really want an Outslayer Muay Thai bag. My basement ceiling is too low for one, even on a bag stand. I've been to every store in my area with a tape measure in hand. No soup for me.