Why do breasts get so large and heavy before period starts? ?

I know its period because this happens to me all the time. Im just curious what it means and i know my periods oming because im moody. Super tired and depressed and fel super bloated with giant breats they popping out of my bra lol. And usually my bras look big on me because i was working out and i was losing weight. Also i noticed by the nipple area its little sticky and sensitive. I had 2 kids before and i know im not pregnant because during pregnancy you ont get systoms like this until almost ue so yh im just caurious ht doe it mean whn it fels like ur boobs jump 2 sizes bigger when ur due for period? Thanks