1.2a, 10, 15, 20, 25 size planes are buy default more sensitive to winds and, depending on several factors are generaly more sensitive to control input. Ground effect also tends to have more effect on smaller aircraft. All this equals faster pilot correction response time. Trainers (pilots) need thought processing time to react so this is really not a good match.

The larger you go the more "stable" an aircraft becomes due to more surface areas available to air flow. Larger planes tend to have thicker airfoils which translates to slower more stable flight etc.

Some would argue the points above but I am comparing apples to apples. small trainer vs largerer trainer of the same design etc.

generaly most trainers start in the .40 size due to the above with .60 gaining speed (so to speak) price and transportability probably has some effect on .40 vs .60 airframes too.

I have really over simplified things but I think you get the idea.

Bill