SAN DIEGO — To become huge again Yoenis Cespedes might have to think small.

As the Mets consider ideas on how to keep their All-Star outfielder on the field and performing near peak level, hitting coach Kevin Long is placing Cespedes’ conditioning at the top of the list.

Cespedes spent six weeks on the disabled list recovering from a strained left hamstring earlier this season and lately had disappeared at the plate — hitting .214 over 21 games entering the Mets’ 6-5 victory over the Padres on Tuesday night. He went 3-for-4 with a double, triple and his first homer in 87 at-bats, but left the game in the seventh inning with fatigue in his right quadriceps.

“It’s almost like he needs to be a little more loose and a little more elastic,” Long said. “I think he’s a little big and a little tight. He needs to get back to the 215 [-pound] Cespedes. I think that’s what his problem is. He needs to stay loose and elastic rather than big and bulky. Because he’s got everything else.”

Cespedes battled a quadriceps injury for most of the second half in 2016 and ultimately landed on the disabled list. He came to spring training bulked up in the lower half — there is video of him lifting 900 pounds with his legs under strength and conditioning coordinator Mike Barwis’ guidance — but that didn’t prevent him from landing on the DL in late April with the strained hamstring.

“Honestly, at first thought, it might have been a good thing [getting bulked up],” Long said. “But his legs couldn’t withstand it. When you are about to play 162 games in 170 days, you can’t be that big.”

In the short-term, Long wants Cespedes back on track at the plate. The Mets saw a glimpse Monday when Cespedes launched a triple in his first at-bat against Padres lefty Clayton Richard before getting intentionally walked twice.

“Every time I think he’s getting close, he goes a little backward,” Long said. “His at-bats have been better and his plate discipline has definitely been better. He is in a good place, though. There is more consistency.”

Cespedes’ slump represents his most pronounced drought since first donning a Mets uniform in August 2015. Entering Tuesday he had not hit a homer since June 23 at San Francisco.

“Everybody goes through it, but we just haven’t seen it and we’re getting a look at it here,” Long said. “Thank God he is actually hitting somewhat decent. The second half has been pretty good since we’ve been back from the All-Star break. It’s a long time. But if you try for a homer you are probably not going to get one.”

Jose Reyes, who is among Cespedes’ closest friends on the team, said there are no external issues hindering the slugger.

“People have to understand, Cespedes, he missed a lot of time,” Reyes said. “He was hurt for more than a month. Before that he was playing not full speed. When you miss a lot of time in this game it’s hard to try to put it together again. But with the talent he has, he’s going to turn it around.”

Long sees a breakout coming from Cespedes.

“We all know it can happen and [homers] do come in bunches, and when he does hit his first I bet it’s less than 10 at-bats before his second one,” Long said. “I have that feeling. His batting practice has been a joke — he has just been crushing balls — so I know the power is there.”

— Additional reporting by Kevin Kernan