Picture two people starting the same January. One decides to overhaul their whole life: new diet, new gym routine, new sleep schedule, new side project, all at once.
The other picks one small goal, like a ten-minute walk after lunch, and nothing else changes yet. The overhaul person has more rules to break and more ways to fail in a single day.
The one-goal person only has to protect a single habit, so a bad day doesn't collapse everything else. Fewer moving parts means fewer chances for the whole plan to fall over at once.
This is the entire argument for starting small: not because big goals are wrong, but because the version with fewer failure points is the version you're still doing in a month.