  Based on the combined model of the double exchange and the polaron, we have
studied the small-to-large polaron crossover transition and explored its
effects on the magnetic and transport properties in colossal magnetoresistance
(CMR) manganites. We have used the variational Lang-Firsov canonical
transformation, and shown that the magnetic and transport properties of both
high and low $T_C$ manganites are well described in terms of a single
formalism. We have reproduced the rapid resistivity drop below $T_C$, a
realistic CMR ratio, and the {\it first-order-like} sharp magnetic phase
transition, which are observed in low $T_C$ manganites.
