Why were nba players bigger more muscular 20 years ago than now? ?

Today's NBA isn't as physical and more players are focusing on 3-point shooting

So you are talking about 2000 and 2020 players?   OK I have been following  NBA since 1996, this is what I think:
"I watched LAKERS in the Final in 2001.   I saw SHAQ, BIG MUSCULAR and AGILE as CENTER.   YES he is muscular and BUFF.    But he was the only player in the NBA who had a physical body like that.   Other center, Tall, lean and slower like Rik Smiths during the time.    Compare it with 2020 center like Dwight Howard, and still the body is BIG, MUSCULAR and less AGILE.    But not as Rim Shocker as SHAQ.  But he is a good center.   So your statement is not true.   There were Muscular centers like Shaq and also the lean ones like RIK.    You have to see the whole payers in the league rather than one team."

Not really true- post players (seemingly) were "bigger more muscular" 20 years ago but guards and small forwards are much bigger now than back then. It seems to have been determined that the best height for basketball is between 6'5"and 6'10" and so we no longer see as many of the 6' to 6'4" and 6'11" to 7'2" players we used to. Similarly the best build for basketball is athletic rather than muscular.

One major reason is college. After Kobe you had a lot of guys going straight to the NBA. Nowadays they only stay for one year. College is where a lot of guys bulk up. They eat good food and have an S&C coach working with them everyday. While they could do the same in the NBA most guys don't. They have the money and freedom and think they're stars. Plus in the NBA they play so many games and are on the road so much that it's hard for guys to consistently get good workouts in. 

It's NOT about steroids. Steroids were popular in major sports in the 1970s. By the 80s they were largely gone because of testing and the fact that they weren't necessary. Better training and nutrition combined with legal supplements could do just as much. On top of that. steroids would make you a worse basketball player by slowing you down and taking away your explosiveness  

if they ate hamburgers as children, the steroids in the food gets transferred to the people who eat them. no drug testing necessary. it's too late.

Easy one this one... Steroids and minimal drug testing