Shaquille O'Neal was one of the most dominant players of all-time in the NBA, with an incredible combination of size, strength, and speed. When he entered the NBA, he was listed at 7'1″ and at around 300 pounds, which enabled him to be extremely agile for his size. As the seasons progressed, Shaq became bigger in terms of weight, which limited his ability to impose dominance as he did earlier in his career. One of the main reasons is that he didn't work out properly and didn't pay much attention to his conditioning and diet.
In the 2003/2004 season, Shaq blew up to 370 pounds, 70 pounds more than in his rookie season. Most players would probably face injuries because of the amount of stress on their knees and joints, but not Shaq. It's quite impressive that he played in 67 games that season, averaging 37 minutes per game.
Even though he wasn't as dominant as in previous seasons, he still averaged 21.5 points, 11.5 rebounds, and 2.5 blocks. That is still pretty impressive, considering there might have been the heaviest NBA history player during that season.
According to Charley Rosen from Fox Sports, Shaq's intentions to get bigger came about after winning the first championship with the Los Angeles Lakers. Despite being the best player in the world at that time, Shaq felt he needed to get bigger so he wouldn't get pushed around in the post.
"It was after the Lakers' first championship that Shaq made a decision that had immediate rewards but proved to be a long-range disaster. Feeling that massive centers like Arvydas Sabonis and Luc Longley were pushing him around in the low post, Shaq felt that he needed to be heavier. In his equation, mass equaled power equaled total domination. He, therefore, instructed his chef to overload his meals with meat and potatoes, and the pounds quickly began to accumulate."
Charley Rosen, via Fox Sports
It's crazy to think Shaq felt he needed to build more mass because of Luc Longley; however, he did gain a lot of weight, which eventually cut his career shorter. It's even remarkable his knees hold up as much as they did, and If he stayed in top shape, he could have prolonged his prime years even more. Shaq's method to get bulkier on purpose most certainly ruined his chances of staying dominant for more years than he did.