Milk in bread dough is a tenderizer . . . typically found in "sandwich" loafs of the "Wonder" style.

If you're getting a really big first rise, then you may have to adjust the recipie with 1) less yeast and/or 2) higher protein flour, if you're not already using bread flour.

It could also be a sign that the dough has not been kneaded enough. Kneading develops gluten which makes the dough tougher. Doughs that aren't kneaded much, will seem wet and provide very little resistance to the gasses formed by the yeast. The result of under kneaded dough range from loaves that collapse or loaves with big bubbles, while over kneaded tends to not rise much and has a denser construction/crumb.