Secure Shell uses Chrome's sync storage so that connection details are saved across devices/browsers/etc...  that storage space is not infinite:
  https://developer.chrome.com/extensions/storage#property-sync
max storage (in JSON string form): ~100k
max number of items: 512

in order to store a connection, at least four items are needed (profile id+desc+user+host).  that means there's at most 128 connections sync storage will allow.  if you add any other details (port, ssh options, whatever), it's going to further increase.  we also store hterm prefs, so that'll further take away from the count.

we could workaround this by using local storage and then manually serializing + compressing things ourselves before stuffing into sync storage, and then decompressing back into local storage, but that would require us standing up our own sync logic between devices which i'm not keen on doing at all.  and at some point, there's still going to be a limit in how much you can save.

we could let people choose to save all their stuff in local storage as that wouldn't have any limits (we request unlimitedStorage in our permissions).  but then they would be responsible for doing any backup+restore of their configs.

i'm going to change this to a feature request as needing 100+ saved connections is a bit of a pathological edge case that most people are never going to run into.

Can sync storage be accounted for in Google Drive quota?

Chrome offers nothing along those lines that i'm aware of

Can other data synced via chromium be reduced in order to make space for
more bookmarks? How can it be seen which data is taking how much space?

Chrome sets a hard per-extension sync limit that is not dependent upon any other factor and cannot be controlled.  if you look at the docs i linked, it discusses this a bit.