I was previously told that for young hamsters, large cages can be incredibly scary and in turn a smaller one for the first month or two, until they are truly settled and comfortable, can be beneficial. Going from one habitat to another is stressful enough, but when you go from one size to something even larger, that can be nerve wracking too. Of course all hamsters are different, but for my Hershey, being in a smaller cage first hand was far better for him. He was more comfortable and more active within his new habitat. After a good few months, and when he simply tells me when the right time is, I'll be moving him across to his new home - The Barney or the Savic Plaza (I haven't decided which one to buy yet aha!)
It's lovely to hear that you've seen him up and about since the change. But yes, keep watching him for signs of distress and see how he goes
