I haven't reared any toads myself yet, all though this year i finally found some bufo bufo eggs.
From seeing the common frogs in my garden, i would say that maybe the tadpoles don't have enough space. Last year, i put some of the frogspawn in the smaller pond, which did have quite a bit of rotting vegetation in it,Initially, there were no predators, so a lot of them became toadpoles. The tads took a lot longer to morph, and were about 5mm. Many of these were really weak, and i found none survived when i raised them. However, in my big pond, Most of the tadpoles got eaten strait away buy newts, backswimmers ect. However, some did survive, and i started noticing some huge tadpoles, which morphed into frogs about 1.5cm, which were much much stronger.
SO overall, it could have been some of or all these things that affected the samller ones:
rotting vegetation
lack of space
lack of predators (so no natural selection of the strong ones)
I haven't tried anything yet on rearing foriegn tadpoles, unfortunately i haven't bred any of my frogs yet
AJfr0ggy