I completely agree that one should learn on small sailboats and that most 40 footers are twice the boat that a 30 footer is. As to handling diffrences, however...
After learning on dinghies
and smallish keelboats boats (under 20''), my first "big boat" training was on a modern moderate displacement 38 footer. After understanding (not yet mastering) how to handle a larger boat (prop walk, windage, current, spring lines
, backing and filling, etc.), I found that handling a 45 footer of considerably more weight was no different. All the skills and principles were the same.
Of course, making a mistake with a 24,000 lb boat is often more costly than on a 16,000 lb one!
In the above, I''m talking about the most challenging handling (IMHO), which is close quarter maneuvering near other boats and docking. Handling in open water under sail seems to be just a matter of higher forces on the control lines