I don't agree. We bought a Silentnight bed and divan for over £800 20 years ago and had to throw it out within 3 years. That's when we bought our Ikea bed frame and mattress. There is quite a difference in our weights but we've never had dips in the mattress.
As pointed out above, Which found...
We've got an Ikea bed and mattress. There are several different types of mattress firmness or softness. The size is European king size which is slightly larger than king size.
We love it and it's the best bed we've ever slept on.