EU exports of steel to the United States in 2017 were worth 5.3
billion euros ($6.53 billion) and of aluminum 1.1 billion euros.
The aluminum imports convert from 1.1 billion euros to $1.36 billion (USD). So, the total value of US imports of steel and aluminum from the EU in U.S. dollars is $7.89 billion. Given rounding errors and hour to hour shifts in exchange rates it would be fair to call it 8 billion U.S. dollars.
So, the EU retaliatory tariffs are being imposed on about 40% of the dollar value of goods subjected to U.S. tariffs on steel and aluminum.
The U.S. tariffs are 25% on steel and 10% on aluminum, so that is about $1.77 billion on a non-dynamic basis (of course, the whole point of the tariffs is to reduce imports relative to domestic production).
Whether or not this is proportionate depends, in part, on the rate charged on the retaliatory tariffs on the $3.2 billion of U.S. goods imported to the EU. Tariffs of 50% would have an equal dollar value on a non-dynamic basis. According to the link above, the proposed EU tariffs appear to be 25% which would be about half the amount of the U.S. tariffs on EU imports on a non-dynamic basis.
So, yes, it is proportional, and indeed a less extreme set of retaliatory tariffs than the tariffs imposed in the first place by the U.S.