"Little toys" sounds like a reference a seasoned flier of larger aircraft may use, long after they have forgotten what its like to fly a trainer.

As far back as I can recall (about 20 years I think) before ARF's were popular and trainers were packaged ready to fly, trainers were about the same size as they are today.
My 1st trainer back then used a Thunder Tiger .35, other trainer type planes I remember having had a Webra T-40 4 stroke, and one had a Merco .61.

.40 sized trainers have become popular due to due convenient size and economy. However, the main problem is that they are heavily built plywood boxes making them heavy for their size and fast to fly. IMO these are not really primary trainers at all.

I agree that the SIG Kadet Senior is a nice looking plane and would probably make an excellent trainer, but many people might not have the space nor the budget for such an aircraft to start.

When I returned to RC about 18 months ago I avoided the plethora of .40 sized 'trainers' available and went for the larger, better flying .60 size.

Many beginners will probably find a large aircraft daunting and would feel more comfortable on a smaller aircraft to start. A small plane can fly well if designed and built correctly.