Spreading loaves

I've been baking a lot of boules and larger (~1 kg) long hearth loaves, and want them to rise higher and spread less. I can't seem to get a loaf more than about 2 inches high.

I proof them in makeshift bannettons (tightly woven long sisal baskets) and the dough is wrapped in heavily floured linen. When I turn them over on the peel, they flatten out a bit, and then flatten more when I slash the tops.

In my last round with the oven, I made one larger long loaf, which I slashed before putting in the oven, and two smaller boules, which I accidentally put in the oven without slashing, so pulled them out to slash after giving them about 5 minutes for their bottoms to set. The boules sprang higher, but I'd still like to see more..

I suspect the following factors hold some key:

1: The surface tension of the unslashed boules allowed them less opportunity to spread outward, then the outer bottom areas cupped the dough when I slashed them so that the remaining live yeast could only expand the dough upward.

2: The round shape maintained an even surface tension in each direction, effectively holding the dough closer to a sort of equilibrium in which the pull of gravity couldn't exploit a longer, straighter side with less surface integrity (the long loaf expanded more to the sides than toward its ends, for example) 

3: less mass in the smaller loaves to be pulled downward and outward.

4: difference in the time between unmolding and sliding onto the stones-- more downtime with the slashing is time with gravity working on the loaf and not heat.

5: Hotter Stone under the boules (lower shelf, closer to my electric heating element).  

6: over proofing.   

7: Under-kneading.   

Any thoughts? Also, does it differently affect the dough to proof in a hard, non-porous vessel like a mixing bowl over a breathable vessel like a basket?

Thanks for sticking with me. Any thoughts?

John