Did a little math using data from my Friends Ranking in Arena. Maybe others who are better in Math can tweak the equation more, but I've gotten pretty accurate results from it.

RATIO = (ENEMY PTS - PLAYER PTS) / 2 / 10000 + 1 Maxes out at 1.5 and minimum is at 0.5

Basically, if your opponent's PTS is greater than 10000 compared to you, you will get a 1.5 Ratio maximum. And differences of 1-9999 will be from 1.01-1.49 accordingly.

At first I thought that the Ratio is based on PLAYER RANKING, however testing it out, it is based on PLAYER PTS. Just as OP said, there is more diversity in the PLAYER PTS of those in the higher RANKS (<3000 RANK maybe?) because of whales spending for orbs. Looking at current weekly rankings now, 1st:51857, 2nd:50000, 3rd 44573. There is already around a 7000 PT difference between Rank 1 and Rank 3. Compare this with PLAYER PTS in lower RANKS (RANK 10000+) wherein one arena win would already give you 500-1000+ boost in RANK, meaning the differences between PLAYER PTS in those RANKS are only <100.

So if we are really matched according to our RANKS, it would be harder for those below to get higher RATIOS. But it would also be faster for them to RANK UP in Arena, because PT differences in the lower ranks is smaller compared to the top ranks.

But I still don't know how the matching is done. I guess RNG is RNG, I sometimes get a match against <1000 RANK even if I'm at >10000 RANK.