Do Biters get weaker further from your base?

I’ve been playing Factorio for a short time and have just developed a tank. I figured it would be worthwhile to explore the zones around my base and scout out potential mineral sites. I travelled beyond the range of my base’s pollution and was surprised to see that biter nests were becoming more massive than those closest to my base.

I had assumed the biters nearest to my base would be strongest, as they consumed pollution, and that biters far away would be weaker – but doesn’t seem to be the case.

Is this unexpected nest population normal? If I travelled further would biter spawns thin out?