I do sympathise with your situation. I remember my parents selling the house I was born in, to move to France. I was devastated at the time - everything I remembered from my childhood was wrapped up in that place. They had a barn sale of things they didn't want to take with them & I went round picking up loads of stuff (which I still have) to fill my tiny flat with, to remind me of the home I grew up in. I always thought I'd move in there one day (total pipe dream of course).
How did I cope? I guess I told myself I had to look at it from their point of view - the house was too big, they could not rent it out after moving, it was a money-pit etc. I think you're already doing this, as you say you support their decision.
The house my DPs moved to was full of their things which made it seem more like home, and that made it a bit easier for me. And I tried to remind myself that it's the people that make a home, not the building. But it was still really hard to feel the loss of part of my childhood.
DPs are now back in the UK as DM's health is failing, which is equally upsetting for them as they don't know if they will be able to go back to the home they made in France. I look at them in their little bungalow & feel sad, but anything bigger would be a heavy burden on my DF and they are comfortable there. I'd rather they had somewhere manageable, close to me, than a big place far away which would make it so difficult to support them. Their happiness was the most important thing.