don't confuse 300VA with watts. "VA" is a different unit of measure, represents total power which is a mixture of both watts and vars (real & reactive power).
the only way a 300VA rated transformer can supply 300W is if the load is purely resistive. all power delivered to a resistive load is given in watts because:
a) the voltage is in phase with the current
b) there is no reflected or shuttled current
so all power is "real" and produces "work" - which in a resistor, means heat.
with motor loads or inductive loads like lighting which is both inductive + resistive, not all of the power delivered from transformer to the load is "real". some of it is "immaginary" or "reactive" which means that the voltage is NOT in phase with the current, or that some of the current has been used "establish the field" or some other strangeness.
so 300VA rated transformer - when applied to a realistic load which in most residential cases is inductive or inductive + resistive .. most of the transformer rating is used by the load to produce work (real power in watts) but some of it is used as reactive power.