Microbial work burns material. DMR reports about 30 per cent weight loss in Phase I and a further 20 to 25 per cent in Phase II. The carbon to nitrogen ratio falls from 25 to 30 at stacking to around 16 to 1, and nitrogen per cent rises as carbon leaves as carbon dioxide.
This is why finished compost weighs far less than the straw you bought, and why nitrogen readings climb without anyone adding nitrogen. Expect the loss, plan for it, and do not read a rising nitrogen figure as proof of supplementation.