Well, they're both pretty large, about 24" thick.
One is a dictionary of medical terms; a family heirloom or whatever, because most of my family (not I myself though) was in medicine.
The other is just a dictionary of the English language.
The English language has more words in it than any other language, but realistically, one recognizes the meanings of circa 50,000 words, and uses circa 5-10,000 in everyday use.
No one really needs an English-language dictionary of more than 30-50 words; the standard Merriam's or Webster's (although the American Heritage Dictionary is my favorite).
I bought the big dictionary when in college, and when reading a biography of Elizabeth I of England, I came across the word "defenestrate."
After taking one look at that word, I decided I needed the largest dictionary possible.
To throw out of a window?
Main Entry: de·fen·es·tra·tion
Pronunciation: (")dE-"fe-n&-'strA-sh&n
Function: noun
Etymology: de- + Latin fenestra window
: a throwing of a person or thing out of a window
- de·fen·es·trate /(")dE-'fe-n&-"strAt/ transitive verb
I love that word!
Panglossian, a view held predominately in DUmmie land.