What was the evidence for bodies much smaller than atoms

The scientist, J. J Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" (electrons) that he calculated as having a very large value for the charge-to-mass ratio. That is, The masses of their bodies (electrons) was much smaller than the atoms but they had a very large charge to mass ratio.

He then proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model.