I'm pretty sure that was a typo because AO3 comments are maximum 4300 characters. 4300 words would be a pretty random, arbitrary cut-off point.
Of course a 1k fic can be amazing, but I'd still be surprised if you don't have more to say about a good 10k fic than you do about a good 1k fic. And that's just the thing - two sentences don't feel like genuine enthusiasm to me. Because if they were really enthusiastic, I'd assume they'd take more than two minutes out of their day to thank me for a fic that I put a ton of work into. It's not exactly a secret that lof of writers love long comments, so unless I know for sure that someone wrote for me who doesn't like them, I will leave a long comment to make them as happy as their fic made me.
Re: Ways in which you are petty
Of course a 1k fic can be amazing, but I'd still be surprised if you don't have more to say about a good 10k fic than you do about a good 1k fic. And that's just the thing - two sentences don't feel like genuine enthusiasm to me. Because if they were really enthusiastic, I'd assume they'd take more than two minutes out of their day to thank me for a fic that I put a ton of work into. It's not exactly a secret that lof of writers love long comments, so unless I know for sure that someone wrote for me who doesn't like them, I will leave a long comment to make them as happy as their fic made me.