Why does this baby look so strong/big?
Excellent observation! By the mid-sixteenth century, when this was painted, European painters both south of the Alps and north of the Alps (including this unknown painter) were falling under the spell of ancient Greek and Roman sculpture. Heroic male nudes in particular established a new ideal for male beauty during the Renaissance for both mythological and religious figures, including Christ. For this reason, the Christ child appears more like a miniature man than an actual baby in this picture.
I encourage you to walk along the wall and compare the many ways that these 14th-16th century artists depict the baby Jesus!