The achevements icon image size is 11*15 while its size in JSON file is written as 16*16, which makes the icon in game is slightly larger than the image.
MCPE-169618
Wrong achievements icon size
Resolved
Awaiting Response
4
(Unassigned)
1188197
Confirmed
Multiple
All
UI
1.20.0.20 Preview, 1.19.73, 1.20.62 Hotfix
Retrieved
Can confirm, affects 1.20.62. To reproduce, simply locate the achievements_icon.png and the start_screen.json file.