Pumpernickel: should it shrink overnight?

My first attempt at Pumpernickel came out of the oven somewhat shunken, so I'm wondering whether that's what should have happened or whether I've done something wrong. I used a German-style recipe, summarised below, from  Daniel Stevens's River Cottage Bread Handbook, with the quantities proportionally reduced by half to a manageable quantity.

On removing them from the oven the next day they had shrunk significantly, almost back to their pre-proving volume, even shrinking away from the sides of the tin. Is this normal or have I goofed up? I won't be cutting into the loaf until tomorrow, so I don't know how taste and texture are working out.

Pumpernickel recipe (Westphalian style)

Soaker: I baked 100g of old wholemeal bread slices until they were very dark brown right the way through, then soaked them with 100g whole rye grains in about 500ml of water overnight.

Sponge: I mixed 150g wholemeal rye flour with 150g water and a good dollop of my wholemeal rye sourdough starter and left it overnight.

Dough: Next morning, I strained and squeezed the soaker in a sieve and retained the liquor, then I mixed the squeezed soaker and the sponge with 125g wholemeal rye flour, 125g rye flakes, 10g salt, 25g blackstrap molasses and 150ml water from the soaker.

Baking: I mixed it all up good and proper, then transferred the dough into two 2lb baking tins so that each was about half full, covered them with clingfilm then let them raise for about 4 hours until nearly doubled in size then I covered the tins tightly with two layers of foil and baked for an hour at 200C then 30 mins at 190C, 30 mins at 180C, 30 mins at 170C then 3 hours at 150C, then switched off the oven and left them in overnight while it cooled. This morning I wrapped the loaves in greaseproof parchment to mature until tomorrow.