How big is XL frame? My son is 6'4", weighs more then heavyweight boxer......ha! ha!
Now, on normal MTB but manage to find those extension for the seat, so still can use....
Different bike manufacturers have different specs, geometry of the bike. Some bike frame builders M may feel like an L to others. The simplest setup for knowing if the bike frame fits you (for normal cross country riding, not downhill racing, etc):
1. Get the handlebars to be almost the same height as the saddle. This is to allow equal weight distribution between front and back - ergonomic standpoint (less stress to wrists or backbone) and balance (front wheels don't lift up when going uphill and don't feel like toppling over when going down).
2. Leg extension - legs should be almost full stretch out with little bend on your knees when seated and pedals at outer end of crank cycle. Feet should be slightly bent with toe ball parallel to pedal spindle. This is to avoid too much pressure/force on the knees and best efficiency.
3. Arm extension - Depending on frame used, bike stem, handlebar and seat adjustment this would determine if your hold is compact or stretched out. If going uphills, tight technical trails, tighter is better. Stretch out is more relaxed for long rides. So need to find a comfortable position.
You can check the following link for sizing measurement as a guide (need to know both height of rider and height to in-seam as it can affect stand-over height):
Bike Size Basics - A Guide to Sizing and Fitting Your Bike
Having a too long seat post tube can be dangerous as it can snap. Need to have sufficient tube length beneath the seat post clamp point too. Too small of a frame with a too long seat post and jacked up handlebar would put your weight distribution to the back of the rear bike wheels which can make handling poor too.
My 2 cents...
