Dint doesn't mean little and the whole idiom definitely doesn't mean "a little hard work". The idiom "by dint of" means "by means of". It could have been an enormous amount of work, e.g. you can say, "He succeeded by dint of an enormous amount of hard work". "Dint" can mean mark or indentation but the original etymology behind this idiom is the sense of wielding a sword - "dint" means a blow or a strike.

Dint means a little mark, or an indentation. In this context, it means "a little". A little hard work. I hope this helps! ☺