As naamanf points out transformers are rated in VA, and as V goes up (110 -> 220), A must go down. The issue is heat
From Transformer Ratings:
"The power-handling capacity of a transformer is dependent upon its ability to dissipate heat. If the heat can safely be removed, the power-handling capacity of the transformer can be increased. This is sometimes accomplished by immersing the transformer in oil, or by the use of cooling fins. The power-handling capacity of a transformer is measured in either the volt-ampere unit or the watt unit."