Weight transformer don't tell us nothing about the power transformer (just give us some clues, but elusive). Leakage magnetic field of a toroidal transformer is much smaller than that generated by a transformer core E + I. Toroidal transformers are made in three standard heights: high, standard and low profile.
For example (weights according to standard SR EN 61558): 1.Toroidal transformers 300VA:
- high profile - 2.97Kg (6.54 lb. lbs);
- low profile - 2.85Kg (6.28 lb. lbs);
- standard profile - 2.92Kg (6.43 lb. lbs); 2. Electrical transformers with core E + I - 300VA = 4Kg (8.81 lb. lbs).
Switching frequency is an important factor in the amount of power a transformer can handle. A very high frequency AC input needs a small core to pass a lot of power. At lower switching frequencies a bigger, heavier core is required. This is why a 500 Watt linear power supply operating at 60\50 hz is heavy but a 500 Watt Switch mode power supply is fairly light weight.
That said, different core materials are required to operate at different frequencies. At low frequencies steel plates (usually silicon steel) can be used. However, at high frequencies eddy currents rise causing excessive power loss, turning the transformer into an induction heater. Typically, at higher frequencies, ferrite is the core material used because eddy currents remain low.