If they have enough light, they will fill out as much as the variety you have can fill out.

Now would be a good time to apply a little Hollytone. I would use it at half the rate on the package just this one time this year and maybe once again next spring.

You did the right thing by opening up the center and allowing air flow. You can assist the filing in by cutting back new top growth so that the lower sections fill in faster.

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Next year, if you have not been satisfied with the response of your azaleas, you really should cut them back to a foot or less from the soil level. Azaleas, as long as they are in reasonable health in the first place, respond beautifully to rejuvenation pruning. Depending upon the kind of azaleas they are, they could grow three feet of lush new growth the very first season.