Increasing the size of a Chapter wouldn't make a difference.
The Imperium's problem isn't the number of Space Marine Chapters, or even the number of Space Marines in a Chapter (in terms of needing an increase; if they decreased, that would obviously be an issue). The problem is that the Imperium encompasses an area of tremendous size, and:
1. ... is simply unable to provide timely, proper leadership and responses to most of its problems. That is, it relies on a hierarchy that the game flat-out states might take years, even decades to respond to a problem... and, when it does, the solution might be even WORSE for the people it's ostensibly meant to rescue.
2. ... it doesn't even control all of this territory.
Between the Warp, unexplored systems, heck, even sectors, hostile alien races, Tyrannids running wild, etc., the Imperium constantly has to deal with what really amount to internal threats, which means it needs to have massive military forces everywhere. That's not often a viable (or even realistic) solution.
What would increasing a Chapter to 2,000 Space Marines do, if they still had the same centralized hierarchy and corresponding logistical considerations?
Now, if you told me that the Imperium was to double its number of Chapters, I'd say that would be a step in the right direction. If you were to tell me ALL Space Marine Chapters had to adopt the crusading model of the Black Templars (and they could either liberally recruit/requisition assets from most planets, or could have Mechanicus assets like forge ships attached to them), I'd say that would be even more meaningful.