Fat experiences

There is not much difference. At 300 pounds you live a normal life and at 400 pounds some things start to limit you.
For me, true extreme obesity starts beyond 450 or 500 pounds.
Walking, getting dressed, getting into a car, bending down, getting up from the floor, getting out of bed, using stairs, etc. The real problems, (health aside), start there.

My wife is just over 350 pounds, but she's just 4'10". That means at my height I'd need to push 550 pounds to match her BMI.

She was at least 150 lbs lighter when we met. (So about 300 lbs on my frame.) We've both noticed that as she got so fat, every little thing in her life starts to get subtly influenced by how huge and heavy she is, and how she needs a little extra time and effort with everything. Getting out of bed, dressing herself, even finding clothing that fits, using stairs. She's quite aware and OK with it, but it is a practical shift in your life once you become so obese.

Getting over 400 was when I started to feel a lot more physically winded and needing to take breaks more often. A good incentive to be lazy