If I understood you right and you want to know how ten zygarde cells/cores turns into a 10% form zygarde, which bigger than ten cells, etc. I can suggest that every zygarde cell/core can produce other cells and cores, but ten cells/cores can't produce more than 10% of zygarde mass because every cell/core have production limit, which decreasing with every generation of cells/cores.
For example: one cell can't produce more than ten cells/cores and every cell/core produced by the first cell can't produce more than nine cells/cores and that lasts until production limit become zero
First cells, I think, appeared like other Pokemon
Hope it helped