The word that began me thinking on this subject has a majority of a letter you wouldn't think would make up more than 50% of any word - the letter "Z." It's not a huge word but an accepted alternative spelling (one I refuse to use, natch) has an even 50% Z's.

I'll close out this thread in a day or two assuming that my Z word is not much of a stumper. "Possesses," at 9 letters, is another goodie. Any others?