Our house is only 70 years old (lived in it for 40 years now) and the same issue can be found in all the lumber. Even back in '41 when our house was built, the lumber dimensions were far different and as far as I'm concerned, the build quality was better. Floor joists are real 2x10 on 12 inch centers with 6" tongue and groove sub-flooring and solid oak on top of that. Very solid.
We had a decent sized addition built to the back of the house not too many years ago, a 14x22 dining room and a full bath above on the second floor. You can tell the difference in the floor bounce just walking into the new room. I'm sure it's fine, but you can feel the difference. Old flooring and lumber was probably still old growth.. not anymore.
To keep things consistent, we had solid oak flooring installed in the addition. Think $$! Back in '41 it was pretty much the standard flooring material.