Originally Posted by Spoonrobot

There's no theory, just experience, measurements and math. Headtubes and steerers are almost 60% greater in diameter, rectangular downtubes/chaninstays and toptubes that are almost the exact same measurement between mtbs and gravel bikes. If you're bored one day, take a measuring tape or caliper to REI or your LBS or whatever and measure the tubes. The wall thickness for almost all aluminum tubing used in bikes is the same - the area of the tube is what matters and where stiffness/compliance comes from. My gravel bike has a 52mmx48mm downtube. This is enormous and is slightly larger than my hardtail from the same year. It's ridiculous.