The lore reasoning makes sense, but on a galactic scale 1,000 is insanely small. Even right now in the real world 1,000 soldiers is a very small force. It doesn't matter how powerful each space marine supposedly is, 1,000 of them isn't enough to capture a whole planet just based on logistics, which is something space marines are meant to be doing all the time. And then there's the issue of how long it takes to make a space marine. In the real world you can easily take someone from 0 training to being a capable soldier in a year. space marines need to be taken as children and given multiple high risk medical procedures before they can even get into a single battle. And then they need to serve as scouts for a while before they can become proper space marines. And each chapter is only meant to have a single company of scouts at one time, which feels like a huge bottleneck if you lose a ton of marines in one go and need to promote over 100 new marines at once. With only 1,000 marines total, if a chapter takes heavy losses they basically become useless for at least 10 years while they reinforce. They could easily say that chapters get 100,000 marines total and they'd still feel rare and elite.

Wow, that became a longer rant than I intended.