True, true...

I can remember once thinking that DZ was always the largest, then either ChiO or TriDelta. Except my freshman year, when we were second. :)

We have a very small Greek system at EWU, but I'm petty sure this is how the order goes:

Sororities
1. Gamma Phi Beta
2. Alpha Xi Delta
3. Alpha Phi
4. Alpha Omicron Pi

Fraternities
1. Phi Delta Theta
2. Sigma Phi Epsilon
3. Sigma Nu
4. Beta Theta Pi
5. Pi Lambda Phi

At San Diego State
 

Fraternities
1. Sigma Chi
2. SAE
3. Sigma Pi
4. Phi Kappa Psi
5. KA
6. Kappa Sig
7. Sig Ep
8. AEPi
9. Lambda Chi Alpha
10. Delta Upsilon
11. Sigma Nu
12. Delta Sig
13. Sammy
14. ZBT
15. Phi Kap

Sororities
1. Kappa Alpha Theta
2. Alpha Chi Omega
3. Alpha Phi
4. Delta Gamma
5. Pi Beta Phi
6. Gamma Phi Beta
7. Kappa Delta
8. Sigma Kappa
9. Alpha Delta Pi
10. AEPhi

Typically at my school, the same 5 or so groups are always the largest- but they usually trade places and are only set apart by a few members.

George Mason University

Unofficial Size Ranking, Largest to Smallest
(this is about 40% fact, 60% estimate)

IFC:
Pi Kappa Alpha
Phi Sigma Kappa
Sigma Phi Epsilon
Kappa Sigma
Phi Kappa Theta
Sigma Chi
Tau Kappa Epsilon
Theta Chi
Pi Kappa Phi
Kappa Alpha Order

PHC:
Chi Omega
Alpha Omicron Pi
Alpha Phi
Alpha Xi Delta
Gamma Phi Beta
Zeta Tau Alpha