Is the transformer leakage inductance increasing or decreasing VS. size
Let's call your small transformer the "apple" transformer, and your large transformer the "orange" transformer.  You would expect that if you're operating at the same voltage but higher currents, that all of the inductances of the "orange" transformer are proportionally lower.  So the individual coil inductances, and the leakage inductances, would go down.

But you're comparing apples with oranges here -- in general this would be the case, and I would certainly expect that if I were shopping for transformers from a manufacturer that cared about consistency and quality that in general inductances would be roughly proportional to rated current divided by rated voltage and divided again by rated frequency.  In fact, I'd expect smaller transformers to be "cheaper" in terms of all of their specifications compared to ideal.

What I would not expect, however, would be that such trends were followed strictly, or that a manufacturer that doesn't care about quality would necessarily do what I expect -- even to the point of delivering product that actually performs as rated.

So -- read the data sheet carefully, trust the manufacturer as far as you can throw their reputation, and if you're working on some one-off project with off the shelf or surplus parts, measure what you've got before you trust it.