Ronaldo always knew he had the talent to succeed in football, but grew frustrated by coaches' comments about his size.
Discussing the difficulties he faced after leaving hometown Madeira to start his time in Sporting's youth system aged just 11, Ronaldo pinpointed where his desire to improve his physicality began.
"I cried almost every day," Ronaldo wrote in The Players' Tribune. "I was still in Portugal, but it was like moving to another country. The accent made it like a completely different language. The culture was different. I didn't know anybody, and it was extremely lonely. My family could only afford to come visit me every four months or so. I was missing them so much that every day was painful.
"Football kept me going. I knew I was doing things on the field that the other kids at the academy couldn't do. I remember the first time I heard one of the kids say to another kid, 'Did you see what he did? This guy is a beast'.
"I started hearing it all the time. Even from the coaches. But then somebody would always say, 'Yeah but it's a shame he's so small'.
"And it's true, I was skinny. I had no muscle. So I made a decision at 11 years old. I knew I had a lot of talent, but I decided that I was going to work harder than everybody. I was going to stop playing like a kid. I was going to stop acting like a kid. I was going to train like I could be the best in the world.
"I don't know where this feeling came from. It was just inside of me. It's like a hunger that never goes away. When you lose, it's like you're starving. When you win, it's still like you're starving, but you ate a little crumb. This is the only way I can explain it.
"I started sneaking out of the dormitory at night to go work out. I got bigger and faster. And then I would walk onto the field - and the people who used to whisper, "Yeah, but he's so skinny?' Now they would be looking at me like it was the end of the world.
"When I was 15, I turned to some of my team-mates during training. I remember it so clearly. I said to them, 'I'll be the best in the world one day'. They were kind of laughing about it. I wasn't even on Sporting's first team yet, but I had that belief. I really meant it."