After the first American Pie, Alyson Hannigan decided to undergo a little plastic surgery and get hot. She had a boob job which increased her breast size to 32B. Before surgery, she was barely an A cup, so this was actually a huge jump. She's looking absolutely fantastic these days and has maintained steady work on the TV series How I Met Your Mother. Here's a pic that shows her in 1999 and the mid to late 2000's. See the difference?
Alyson Hannigan took a super conservative approach which yielded absolutely fantastic results! After the plastic surgery, she looks totally natural. Her breast implants aren't even that indicating.