The ICE index (aka USDX) was developed in 1973 based on what was then a trade-weighted model. It has been updated once, when the Euro was created. As a result, the Euro accounts for more than 56% of the index's weighting.
By contrast, the Dow Jones FXCM Dollar Index (aka USDOLLAR) gives equal weighting to the following major currency pairs: EUR/USD, GBP/USD, USD/JPY and AUD/USD.