Amrafel
Well, you notice the border tight around the actual captcha? The one that looks like it's made out of sticks.
It's designed and coded to fit these relatively small captchas. Other, more readable captchas, are larger. You see, the captchas themselves are generated by a third part, not gaia. To change captchas, they'd have to go in and fiddle with the game code. Possibly breaking the game forever.
Just keep plenty of calming medicine nearby when it's time to save a bucket.
not all readable captchas are larger. marapets, a site i play very rarely, has small nice and neat captchas, smaller than the fishing ones. I am not sure what captcha system they use though or i would have just named it and not the site.