Although from starkly different parts of the world, Reynaldo Lopez and Lucas Giolito have become friends over the past three years, teammates at short-season Auburn, low-Class A Hagerstown, high-Class A Potomac and now Class AA Harrisburg. When they line up for the national anthem before games, Lopez sometimes stands near Giolito. Both top pitching prospects have fastballs that have been clocked in the upper 90s, but Lopez is generously listed at 6 feet and Giolito perhaps conservatively at 6 feet 6.

Still, Lopez mutters to himself: “Man, this guy is big.”

Flame throwers come in all sizes and walks of life. Giolito has always been tall, grew up in Los Angeles in a Hollywood family, was a first-round pick and signed for $2.925 million. Lopez, on the other hand, grew up in San Pedro de Macoris in the Dominican Republic, didn’t sign with the Nationals until he was 18 (later than many top prospects), accepted a signing bonus of $17,000 and had to develop into the high-octane pitcher he is now.

“It’s crazy,” infield prospect Wilmer Difo said of Lopez, a friend, Harrisburg teammate and fellow Dominican. “I don’t know how someone his size can throw that hard.”

Lopez was born with a strong arm. Normally a catcher in little league in the Dominican, he filled in at pitcher one day, threw hard and his coach convinced him to stay on the mound. Lopez said his uncle played baseball and had a bazooka for an arm so, “it’s genetic.”

Unlike many Dominican prospects, Lopez left baseball at 11, dedicated himself to school, completed his high school degree and returned to training at 17. Many top prospects sign at 16; Lopez did so two years later because of school. Known for finding bargains and signing relatively older players, the Nationals saw potential in Lopez and offered him $17,000 after a tryout in 2012.

“It’s not that much,” Lopez said recently in Spanish. “But when you want to do something, the money doesn’t matter. … I called my mom. She said, ‘Take it. You don’t know if this is the opportunity God is giving you. Don’t waste it.’ I took it and mom said, ‘Don’t give me anything. I trust in you. I know you’re going to make it to the big leagues.’ ”

The next few years were filled with growing pains. Lopez’s low to mid-90s fastball dropped to 88 mph then jumped back up later in the year. He weighed only 160 pounds then. A coach offered advice: invest in your body. So Lopez used vitamins, drank protein shakes and lifted weights — all new to him because his training wasn’t as sophisticated in the Dominican.

The velocity jumped back up as Lopez added muscle. He said he once hit 99 mph in extended spring training. But in 2013, his arm bugged him and his velocity dropped to the high 80s again. He hadn’t thrown that many innings before and, according to Lopez, doctors told him he had weak bones. He essentially missed the entire season rehabbing, building strength and drinking milk.

“I didn’t like milk before,” Lopez said. “Maybe that was the problem. Then I started drinking milk and two things of calcium daily.”

Lopez said his first pitch of the 2014 season was 98.5 mph. He topped out at 101 mph. He jumped from Auburn to Hagerstown and Potomac, and up the prospect rankings, too. He posted a 1.30 ERA over 83 1/3 innings. Last season, Lopez took a step backward, dealing with inconsistent mechanics and a back issue, and finishing with a 4.09 ERA in 99 innings.

At Harrisburg this season, Lopez is again working toward consistency against better competition. He has a 4.60 ERA through six starts entering Wednesday’s games. He struck out a season-high nine and allowed one run over six innings Wednesday. In a recent start, his fastball sat at 96 mph and hit 99 mph. He is learning to control his velocity, maintain aggression and improve his command.

“I always thought in my mind: you’ve got to throw hard,” Lopez said. “[Nationals senior advisor for player development] Spin Williams told me that’s not pitching; that’s throwing hard. You’re called a pitcher, not a thrower. So he told me, ‘You have to learn that. And when you want to do more, you don’t get it. When you do less, you get more.’ ”

Baseball America rates Lopez as the Nationals’ fifth-best prospect. The Nationals view Lopez as a starter, but some evaluators believe the right-hander could end up in the bullpen, perhaps as a 100-mph-throwing late-game reliever. Lopez said he loves starting but will do whatever the team asks of him.

“I think he’s going to have great success as a starter,” General Manager Mike Rizzo said in spring training. “That’s not to say that early in your career – we’ve done it before and a lot of teams have done it – you cut your teeth in the big league as a reliever. It’s not out of the question, but we see him getting his reps as a starter this year in the minor leagues and he’s got a special arm. He’s a top-notch prospect and we’re looking for big things from him in the not-too-distant future. He’s that next wave of good young arms that are coming.”

Lopez and Giolito are part of that upcoming crop of pitching prospects. The Nationals are excited about the potential of both arms but believe they still have much to learn before reaching the majors. Even though they have different backgrounds, Lopez said he has learned from Giolito, and vice versa. They love watching each other pitch.

“It’s not the size,” Lopez said. “It depends on how you work. It’s a gift of God. A lot of people work hard and don’t throw that hard. You have to give thanks to God for that arm.”