Samosa dough shrinks during baking primarily due to gluten elasticity and the release of internal moisture, causing the dough to contract.
Samosa dough, like many pastry doughs, contains gluten. Gluten is formed when flour is mixed with water, creating an elastic network. When the dough is stretched and shaped, the gluten strands are also stretched. During baking, the heat causes the gluten to relax and contract, pulling the dough back towards its original, smaller size. This is especially noticeable if the dough hasn't been allowed to rest sufficiently before shaping, as the gluten will be more tense.
Additionally, the heat from baking causes moisture within the dough to evaporate. As the water turns to steam and escapes, it reduces the overall volume of the dough, contributing to shrinkage. The rate of evaporation and the amount of moisture present also play a role in how much the dough shrinks.
Finally, the type of fat used in the dough can also influence shrinkage. Fats like ghee or butter, when melted during baking, can create steam and contribute to the dough's contraction.
Allow your samosa dough to rest for at least 30 minutes (or even longer) after kneading and before shaping. This allows the gluten to relax, reducing its tendency to shrink during baking. Covering the dough while it rests prevents it from drying out.