We've all been there. You sit down to write a "simple" patch. Maybe it upgrades a module to Drupal 6. Maybe it adds a small feature, or fixes a bug that's been annoying you for awhile.
You grab your caffeinated beverage of choice, pop open your text editor/IDE, and throw on your coding tunes. You search through the file for the lines you need to change and... sayyy... You know, the indentation is off a bit here. I'll just go ahead and fix that. Oops. And this function's PHPDoc is missing. No problem, a couple quick lines here. OH! That's clearly a bug. Better fix that while I'm at it. Hey, and that reminds me! I always kind of wished that this particular feature worked a different way. Might as well pop that in while I'm at it...
Suddenly your "simple" patch has grown into 30-40K of changes. It fixes the original thing you set out to fix, and it also makes several improvements to the code that you found as you were going through. Great! So what's the problem?
The problem is context switching.
It's hard work doing a good, solid patch review. You need to browse through the code and look for any weirdness. You need to download and apply the patch to your test site (and make a test site, if you don't have one yet). You need to actually test the changes and make sure that they do what they're supposed to do. And, if you're talking about Drupal 7 core, which is my life now ;), you also need to run the test suite, or at least the tests that you know for a fact are affected by the patch.
When you upload that horrendous, multi-headed, kitten-eating monster of a patch, suddenly that 15 minutes between phone calls isn't sufficient to get the patch reviewed. Now reviewers need to find more like 45 minutes or an hour between things, which is much harder to come by during the run of a day. Parts of the patch that make API changes that need careful attention and discussion are lost in the noise of minor whitespace fixes and other things, so the chances of not catching problems ahead of time is much higher. And the individual parts of the patch don't get ample attention, since they're mooshed together in one big glomp of diffs.
So please, when you upload patches, try to keep them to one "context" at a time (feature X, bug fix Y, minor cleanup, etc.). More patches will get committed sooner, more reviewers will help out, and kittens will live happily uneaten.