Core losses of a transformer are considered fixed. I have never seen a calcualtion that varied the core loss in realtion to the loading.
Where did you come up with only three differences in type dry type transformers? The construction features most likely to impact core losses that I am familiar with are:
Aluminum or copper conductor
Shell or core core construction
Butt, wound or mitered core
Standard, high, or energy efficient design
150, 115, or 80C temperature rise
