I'm not sure why Pixelfed's would be shorter than Mastodon's, maybe someone at @pixelfed can answer?
There's usually always some kind of limit though?
Federating just means the servers can cope with linking up to other servers, they may not necessarily have identical features. (For example some servers have character limits in the thousands, and these will display correctly on Mastodon, but Mastodon usually restricts posts to 500 characters.)