Let me tell you, it's going to be OK! The vast majority of children DO potty train out of their one-size diapers, even if they were a super chunky baby! Let me tell you my story :) Ian, my oldest son, came into the world more than 4 years ago at a whopping 9 pounds 11 ounces (born at 39 weeks, yikes!). We didn't start cloth diapering him till he was about 12 weeks old and we exclusively used BumGenius 3.0 One-Size Pocket Diapers. Right off the bat, Ian was on the second largest setting and within a couple months, they diapers were fully unsnapped. I was FREAKED! Did I just drop hundreds of dollars on these diapers only to use them for a few months?!
Then, he started to crawl and slimmed down a bit. Then, he started to walk and slimmed down even more. He was never an incredibly rolly polly baby, just really solid. Their body shape changes as they grow and even those who have maxed out the rise of the diaper will likely still be using them well into toddlerhood! I've also found that the more mobile baby becomes, the easier it is for them to have a lower rise diaper. As the close in on potty training, it more resembles underwear then, too! So, the good news is that when he potty trained just before his 3rd birthday at about 32 pounds, he was still fitting beautifully in his one-size diapers and we had actually snapped man of them down a snap to get a more trim fit under his toddler sized jeans. Just a couple months ago, our family was hit with a stomach virus and at 4 years old and 40 pounds, I put Ian back into one-size diapers for a couple days without any fit issues!
Pictured above, (left) Ian at 7 months old with the diaper fully unsnapped and (right, below) Ian at 2 1/2 years old, still wearing one-size diapers! So if you're like me and many of the concerned parents who's little one seems to be growing at a rapid rate, take a deep breath! You're not the only one to be having the "First Year Freak-Out!"!