Came across this and had that shock of recognition - a childhood read and the illustrations brought everything back.

I realise that sounds a bit heavy for kids, but it makes them and you think - what would happen if the UK were strictly segregated along lines of weight? The fattypuffs and thinifers duke it out in a war for supremacy, eventually signing a peace treaty and agreeing to live side by side. It concerns the imaginary underground land of the fat and congenial Fattypuffs and the thin and irritable Thinifers, which is visited by the Double brothers, the plump Edmund and the thin Terry. It is accepted by you that Daunt Books has no control over additional charges in relation to customs clearance. Surprised and delighted to find this book still available - a teacher read it to us way back in 1950ish and I see now that it has political overtones which escaped me when I was a child.

Apparently -- and obviously, in retrospect -- this is an anti-war allegory, which completely passed over my head.

Thoroughly entertaining fantasy about two brothers plunged into a war between a nation of obese pleasure-seekers (the Fattypuffs) and a country of skinny grumps (the Thinifers). When brothers Edward and Terry find a staircase leading down between two rocks little do they know the adventure they are about to embark on.I felt it did a very good job of showing how some things such as size discrimination can be so silly. I realise, incidentally, that the whole notion of the book would probably be greeted with horror by the health and safety (and discrimination) brigade – almost as much as some of the other favourite books of my childhood which I do not even dare to name! Terry also rises through the ranks quickly in the land of the Thinifers, workaholics all, who scarcely eat, and who rush to and from their country, which is all high, sharp spires and thin railway cars. Consequently, the Thinifer president proclaims that the two peoples form a new nation, the United States of the Underground.