Weight divisions

The UFC did not utilize weight divisions until UFC 12 on Feb. 7, 1997, when a heavyweight division (more than 200 pounds) and middleweight division (less than 200 pounds) were instituted. Part of the UFC’s original allure was to provide a venue where a 400-pound sumo wrestler could compete against a 100-pound karate expert. It was not abnormal for the smaller combatants to defeat larger opponents. Such victories were often due to the mastery of one discipline that worked well against another regardless of size and strength.
UFC Weight divisions

As fighters developed multiple skills and masters of single disciplines gave way to multi-disciplined athletes, it became necessary to develop weight divisions. As in boxing, it is unsafe and unfair to match equally talented competitors of drastic strength differences together in competition.

The lightweight division (less than 170 pounds) was added at UFC 16 on March 13, 1998. It was not until UFC 31 (May 4, 2001) that many weight divisions were instituted as a result of the New Jersey State Athletic Control Board involvement in developing uniform UFC rules. The new weight divisions are:

Flyweight Under 125 pounds
Bantamweight 125-134.9
Featherweight 135-144.9
Lightweight 145-154.9 (Jens Pulver )
Welterweight 155-169.9 (Carlos Newton)
Middleweight 170-184.9 (Title Open)
Light Heavyweight 185-204.9 (Tito Ortiz)
Heavyweight 205-264.9 (Randy Couture)
Super Heavyweight over 265