In my most recent runner hatch (last year), one of the ducklings was like that, WAY smaller than everybody else. Probably something in the genes that just happens once in a while?
At the top of the page between where it says "rules" and "MyPage" it says search. Click that and for keyword write "microduck." Where it says "where to search" drop the tab and click "Ducks." Click "Submit." You'll get the relevant threads. Start at or near the bottom of the results and begin reading. We love Microduck around here