If it's 2X or 3X, it's just adding 30% to 40% body weight. For 4X or 5X, it's like adding 100lb to an 150lb body. We have plenty of people in the 250lb to 350lb range and they have no problem in mobility. Human body can handle the load. It might still be helpful to have exoskeleton help.
You'd probably still have issues if you tried to live there. Your legs can support twice your body weight if they need to, but can your neck support a double-weight head? Human spines are already pretty under-spec for their task, so people living in 2g gravity would probably suffer from a lot more back problems.
It's a pretty small-beer problem on the scale of colonise-another-star-system problems though.
Not to mention that large humans do take time to become large. A 200 pound man doesn't wake up one day weighing 340 pounds. Yes, a human body can adapt to it, massive health problems notwithstanding, but that doesn't mean you can step off a space craft into that situation. To say nothing of doing so after a multi-generation trip through bone- and muscle-wasting space.
