I'm posting this after reading the answers to this: Where is the Signature stored in a SegWit transaction?
I've found out from reading that, that in Segwit, the "witnesses" (or signatures) that verify a transaction's authenticity are still stored on the blockchain. In which case, the same amount of bits ought to be used up by a given transaction as if Segwit weren't being used. If the block size remains the same, let's say 1MB, then it ought to hold the exact same number of transactions.
I've got an alternative possibility. Let's say that what actually gets sent to the nodes is a transaction with an empty signatures field (I think that such a field doesn't quite exist because of multi-sig stuff and scripts, but bear with me). Then the signatures can be sent through a separate channel. What actually gets stamped onto the blockchain is a transaction with an empty signatures field. And then because of the longest-blockchain-always-wins rule, the non-Segwit nodes will simply accept that the blockchain with the most number of transactions, which includes some funny signature-less transactions, is the authoritative blockchain.
But apparently reality is more complicated.
Also, there's a question about how much the longest-blockchain-always-wins rule can be abused. (The word "abused" does not imply a negative opinion). For instance, let's say I come up with FoobarWit. FoobarWit is the same as Bitcoin but with a larger blocksize (no Segwit). Once enough nodes adopt FoobarWit, then more such nodes will win the mining race, and so other nodes will effectively accept bigger blocks because the history is longer. I know that's not the case, but why?