Hi Quentin, thanks for all those graphics and don't want to deviate the thread, but even when the saddle/handlebar distance is the same, the pedaling position it is not. In this example you are much more over the BB which I don't want to change from my actual bike as I train on three different bikes and have it dialed in terms of power and comfort, this is why I think it would be easier for me to get that position on a M frame. I would like to keep the same saddle to BB height/offset and distance to the handlebar. Maybe it would not be possible on a L as the reach is much longer.
Just curious, what are your other 2 bikes? Here's some potential perspective. Take the example that you get a bike fit on a road bike, the fit is done on a stationary trainer. There will be an ideal saddle height, set back, and tilt to maximize power and minimize discomfort. This is important because you are always seated in the same position and grades are not as steep as MTB trails. For an MTB, the "perfect" pedal position is less important in general because you're constantly moving around and shifting your weight, therefore the best position is a balance between good fit and weight balance. Now what if you redid your bike fit, but you lifted the front tire up enough to match a 5 or 10% grade? You'd push the saddle forward to compensate, since the STA gets slacker. It's true that when you go back to flat ground, it's a compromised position, but if your trails are constant up and down, does that matter? Take this with a grain of salt, the new style geometry isn't for everyone. I'd bet that a bike with a 69 deg HTA and 73 deg STA would be faster on flatter XC trails than the newer bikes with 66 deg HTA and 76 deg STA, especially tight courses. But if that's what your trails are like and you want the perfect pedaling position on flat ground, then this likely isn't the bike for you. If you downsize and push the saddle back, you'll just end up with a rearward weight balance, it'll be hard to weight the front tire in corners and won't feel right. The slack HTA, steep STA, long reach, and long wheelbase all play together.