I've read that growing breasts during pregnancy is a healthy indication of future milk supply, true? the bigger the breast, the more milk?

Not exactly: Breasts have gland tissue and fat tissue. Gland tissue is what supplies the milk (makes the milk and temporarily stores it). The fat tissue helps give the bigger breast size, but doesn't make milk. Normal gland tissue in breasts big or small can make enough milk at a steady rate for their babies, and make more if a baby drinks more. Some breasts do store more milk so meals can be farther apart.