Right now, it's basically a 1:1 relationship in the long term for a very basic reason. According to moneroblocks.info, there have been roughly 8100 transactions in the past week. Since there are 10080 minutes in a week, that means there have been approximately 8 transactions every 10 minutes.
I'm going to go out on a limb and say this have been a higher-volume week due to the big price spike and news, and use a smaller number of 5 transactions every 10 minutes for the rest of my answer. Why 5 every 10? Because for a two minute block size, that means 1 transaction every block on average.
The on average bit is important. Monero has a two minute block time, but blocks aren't always exactly 2 minutes... they just aim to average to be 2 minutes. Sometimes a block takes 10 seconds, other times it takes 10 minutes. And this is the whole crux of why longer blocks seem to have more transactions.
Basically, if we see a transaction on average every 2 minutes, and we get a number of 10 minute blocks, we should see approximately an average of 5 transactions in those long blocks. Likewise, if we see a number of 1 minute blocks, we should see approximately an average of 0.5 transactions per block.
Disclaimer
This is all based on the assumption that transactions are basically included in blocks right away. At very heavy usage times, we could see a backlog of transactions like Bitcoin has at times experienced, and as a result see a bunch of blocks in a row that all have similar large numbers of transactions regardless of how much time has passed. This gets muddied by the variable block limits in Monero, but is beyond the scope of this question.
