Upvote:3
A number of new UK trains have thinner padding on new seats in order to increase legroom - people are taller now on average and need more legroom, but the trains aren't getting any bigger, so they can get two or three centimetres by making the padding on the seats thinner.
I don't know if this is Eurostar's reason, but it is the official explanation for the thinner padding on the new Thameslink trains ("class 700") compared to the old ones ("class 319").