What makes you think it's not compressed? (or that the data is stored as XML?)

There's very sophisticated compression systems throughout each experiment's data acquisition pipelines. For example this paper[1] describes the ALICE experiment's system for Run3, involving FPGAs and GPUs to be able to handle 3.5TB/s from all the detectors. This one [2] outlines how HL-LHC & CMS use neural networks to fine tune compression algorithms on a per-detector basis.

Not to mention your standard data files are ROOT TFiles with TTrees which store arrays of compressed objects.

It's all pretty neat.

[1] https://arxiv.org/pdf/2106.03636

[2] https://arxiv.org/pdf/2105.01683