That's quite an obvious statement isn't it? The one in the title. Well I didn't learn this until recently.
Let me explain myself, and how I'm not dumb.
A small project you can get a skeleton for in like an hour, you spend a bit of time brainstorming features and how you can simplify it and in an hour you have a skeleton, in 15 mins you have a project ready to start working on, you've figured out the README the license etc., you just have to start making it now, and that takes like 3 hours or less if you're good, depends on the project of course.
In comparison, a big project you don't plan out quite as much, since that's a bad idea to do at the start, instead you setup some tests (if you work Agile), you make some decisions on what to start with etc.
After 5 hours, you're not nearly done with the big project, while the small project you're at least halfway there if not done already.
If you're used to small projects, like I am, a bigger or slightly bigger project may make you feel slow, at least it made me feel slow.
I just kinda want to say that's probably normal and not something to worry about, if you haven't finished your idea in 6 months, maybe start worrying. lol