I always wonder if fossils preserve the original size of the former animal. I can easily imagine that trough pressure the original animal is compressed and appears smaller as a fossil afterwards.
The 'filler' usually isn't sedimentary - it tends to be formed by something that seeps through then crystalizes.
Those crystals are super hard, and generally won't be compressible at all.
Although there is a good chance the cavity compressed a bit before being filled.
https://www.nps.gov/pefo/index.htm
Straight from the Triassic.
Ed: to clarify my first two sentences, when you see a flattened fossil, IIRC they were already flat before they turned to rock, by relatively light forces acting on the body. It wouldn't spread out sideways unless the whole rock it was fossilized in was later squished by geologic forces.