Some smart leather-working should save your priestess from flashing.
This is so much easier with modern materials. Velcro or spandex would make this so much simpler. But, alas, medieval tech it is.
For the chest, torso, waist, legs, and arms, you basically have an expanding cylinder - I'd build a flexible outfit out cotton or linen or hide that's much too large, and fold it back on itself, like this:
If the transformation is rare (warewolf once a month sorta thing), or the smaller being needs to be athletic and mobile, then a few sacrificial stitches could hold the garment taught on the smaller user, and then immediately give way when the transformation occurs. Those sacrificial stitches can be repaired in a few minutes by the small being after the change, they'd probably keep the needle and thread on them.
However if the transformation is very common, and the small being doesn't need to do gymnastics, then a thin peice of rope wrapped around the outside, granny knotted and the ends tucked inside will do the trick.
However, if the transformation is very common, and the small being needs to be extremely mobile, then you need something with two latching positions.
Cut these two shapes out of leather:
The long peice of thonging with a circle in it attaches one side to the clothing, the other end hangs loose.
The Double-T shape rolls up into a cylinder, with the T bits inside (which result in that part of the cylinder having smaller diameter, smaller than the circle). It's then stitched shut, and one end is attached to clothing.
The leather is flexible and distorts under strain, but the circle in the leather thonging is held against the T bits under normal forces.
When the character expands, the forces are much greater, and the leather distorts and travels to the next position.
After they transform back, they pull the string hard and their clothing tightens, the circle then travels past the other T, and is then held taught again.