There are a couple answers:
1.Biology:Males are attracted to females. More exactly men are attracted to females that physically show they are good for having children. The whole point of attraction is sex and the whole point of sex is to have kids and keep the human race alive, biologically speaking. Big breasts are a good sign of 'this female can care for kids'(biologically speaking as per breast feeding.
2.Difference:The whole reason why guy are attracted to girls and girl are attracted to guys is their differences. What you see is not what you see in the mirror.
3.You have seen popular culture, right. Big boobs are everywhere. Guys, from age 0 are told that big boobs are good and cool and desirable. Watch any TV show, and you see big boobs. Watch the news and you see big boobs. Ads are full of them too.
4.And of course boys learn about big boobs from their parents and family members too. When 'aunt Trudy' talks about getting a boob job, dad watches big boobs, mom complains about her small boobed figure and on and on. This stuff sticks in the boys head.
5.Big, big boobs are rare. Pick out 10 girls and you will get all different boob sizes, but only a couple big ones. Anything rare is good.