Compare Medium Diaper Covers
Snap Version 
Hook and Loop Version
old style hook and loop Thirsties Duo size two on a 14 month old 19 pound toddler. 
Snappi fastened size medium red edge Cloth-eez prefold diaper is underneath. Nothing to fold down. It fits perfectly.
Middle rise setting over the Snappied medium red edge Cloth-eez prefold on 14 month old 19 pound toddler.
Thirsties Cover Size Medium, hook and loop version
At 14 months and just 19+ pounds, she is on the thin side. This (old style aplix) Thirsties medium Cover works on her, but it is "full" in the rear for her and a little wide for her. It will work fine, but for this baby, the Duo is a nicer fit. A fuller bottomed, heavier, wider baby would fill out this cover more. There is plenty of growing room in this cover, and plenty of room for adding doublers to get lots of absorbency if that is needed. Notice the hip area in the middle picture. The regular sized Thirsties covers are tall at the hip and fuller in the rear than the Thirsties Duo or Bummis Super Whispers. It does fit her with growing room, but I'm just trying to point out the subtle difference in the cut and fit of various covers for you. We stock only the Thirsties Cover in snap in all sizes (we no longer carry the medium pink one shown). We do carry the Thirsties Cover in hook and loop in size large, since that is the next size up from the Thristies Duo hook and loop cover size 2.
Bummi Super Whisper Wrap Size Medium. This has covered PUL inside and no gusset.
When a diaper cover is too small, it's not as comfy as when it is roomy. If a cover is too large on baby it will gap and leak, but too tight isn't good either. A good fit is important.