Well...My signature looks bigger than yours does, because it's longer. I couldn't make yours 550 by 134 because it would have thrown it out of proportion. With text, you can have a signature of 600 length by 134 height. Without the added text you can have a signature of 600 by 150. I would say, most people have signatures that are 134 x 550, which is what mine is. I also make all of my original signatures at that size, just in case someon wants to add text.
If you wanted to re-do your signature, you can make it at 134 x 600, and as long as the bytes are less than 30. Your signature will be acceptable here, or if you remove the text you can make it at 150x600
We normally notify a member of an oversized sig when we see it. It's possible that no one has seen the ones you are referring to . Feel free to contact a Mod in the future if you see one that is oversized.
I doubt there are many sigs over the size, there are, at times, a few people who have been contacted and given time to change theirs and some the team may have missed, which as Karen said, feel free to report.
But if you look at signatures without checking the actual sizes, they can look much bigger or smaller. Karen's looks bigger than mine for instance, because of the lighter colours
I know I have looked at a sig and clicked on the properties to PM them asking if they need help resizing and while I thought it was too big, it was fine when I looked at the properties.
For you and anyone else who is unsure, I made a template for the size of the sig
This is the very max size a sig can be.
However, at this size, when saved to less than 30kb they can look low quality so people make them smaller, which also allows the user to have a line of text, as shown here
(white area shows max size including user text, pink/purple with black border shows size of typical sig)