The only way to get more results inside the radius is to "increase" the size of the radius, not half it. And doing so only creates an illusion that the search API just worked.
Radius is completely ignored. No amount of changing the size of it will effect the 100 tweets that will be returned. You will still receive the top 100, most recently posted, tweets from the placeId your lat/long is inside of.
Try this query:
If the API was working you'd see all 100 results hit within that 1 mile search, all tweets originating from me and my co-coworkers, as we have tweeted far more than 100 times over that exact spot for the last few months. You probably won't see a single one though, instead getting 100 tweets from other areas all over the city of Grand Rapids. If you see any tweets at all, look at the time they were posted, it will be a few minutes ago. Check back later, they too will be gone with the rest of them, never to be found again by a geocode search, because there are 100+ more newer posts in the general area outside the radius that will be returned first.
If I know of 80 tweets that do indeed exist on a certain location, doing a geocode search of that location should return exactly those 80 tweets and only those 80 tweets (barring drop off of ones older than a few weeks, or whatever that cut-off is set at for Search results).
100 results outside the radius, the API is not working.
5 of my tweets inside the radius and 95 others outside the radius, the API is not working.
Hell, even all 80 tweets showing up in the results and 20 others outside the radius showing up still would be a bug... but I would gladly accept those results since I can always filter out the unwanted 20.
Using my location, Grand Rapids, MI, as an example....a geocode, lat/long, search, currently does nothing more than the following:
* You're lat/long is near Grand Rapids.
* Return the all tweets from Grand Rapids, sorted by most recently posted.
That's it. Posts falling inside the radius are purely coincidental. I'll get my latest tweet, not because it's inside the radius, but only because it is the most recent tweet in all of Grand Rapids.
I don't know if any of this helps or not. I've repeated a lot of this in a few discussions, but have only ever seen responses that look to me like results of the illusion I first eluded to at the beginning of this little book of a post I just wrote.