1. A transformer transforms both voltage and current. If the transformer ratio is different from 1:1, input current (the current drawn from the mains) and output current (the current consumed by the load) are also different. If we neglect magnetizing current and transformer losses for the time being, the currents can be simply calculated Pout = Iout*Vout = Pin = Iin*Vin.
2.If you look at the specification of a stabilizer transformer, you probably find different ratings for input and output current, or may be both are the same. If you determine the available output current for a specific voltage ratio setting, you have to check both ratings. In boost mode, it's likely that the input current is reached first. In other words, the higher the boost ratio, the lower the available output current.