The multiverse is maybe as big as the universe but still has more universes inside?
How does this happen? How do the universes shrink inside the multiverse?
You have the wrong concept, but I will try to approach this in a manner you might be able to picture.
First off, a definition of "verse" is "turn", "turning." Universe is essentially one turn, one turning, to turn, or the turning. So I will approach it that way here.
Stephen Hawking conceptualized a single particle having six faces to it. As it is [[turned]] it displays each different face as a different particle. Six individually different particles; one individually different particle per each of six faces in the "turning" particle. Taking it up scales, a universe in one turning (uni versus), yet at exactly the same time a multiverse of six universes.
Redundantly, the turn of the Universe could yield an infinity of different faces in the turning (the verse). An infinity of individually different universes in the Multiverse of the Universe, including even in the multiverses of those infinity of universes. You could say a form of "fractal self-similarity" at work.
The particle-like multi-dimensional look of many in one "turning" is not someone's imagination, unless it is Stephen Hawking's among many others who dealt and deal in particle physics. A multiverse can have many different spokes (roads), many different looks (scenarios), many different dimensionalities (many different beings), all leading essentially into and out of the same hub of all. To think that there is only one road, only one way, and that everything else is imagination is quite wrong.
Even concerning Relativity, relativity predicts its own breakdown. Relativity is lost and gained, lost and gained, lost and gained. It opens and closes, opens and closes, collapsing in horizons as new horizons acquire. Thus, universe (singular) can and will offset into universes (plural). An infinity of offsetting relativities also being an infinity of offsetting universes; also being multiverse.
It is not imagination that relativity breaks down, or that one of a thing can divide (fission) into two... or many... or an infinity.