Lower efficiency with smaller batches?

I've been brewing all grain for about 8 months now, approximately 20 batches (@ 5 gallons). I've had OG's ranging from 1.040 to 1.190 and have always hit around 75% efficiency.

Recently however, I've been brewing smaller batches (2-3 gallon) and my efficiencies have gone down pretty substantially. The last 2 small batches I mashed only hit about 62% efficiency, but I haven't modified my process. Could this be an issue relating to lack of enzymes? Is diastatic power calculated cumulatively (IE does it go down with smaller grain bills/batch sizes)? Thanks for your ideas!

My first all grain was 2.5 gal, and I got 83% efficiency with BIAB.

I've found that equipment losses become more pronounced with smaller batches. Maybe you leave a quart of wort behind in the boil kettle, mixed together with hops and coagulated protein. Well, in a 5 gallon batch, that quart isn't missed nearly as much as it is in a 3 gallon batch.

Was the crush the same? What was your process?

I doubt it has anything to do with diastatic power as that is more a function of percentage than amount. If you aren't changing the any of the grain or water ratios, that doesn't come into play.