As we learned from Are Katy Perry's breasts real?:

If you notice the top of her breasts are not nearly proportionately as full as the bottom portion. So what does this mean?

When the bottom of the breast is considerably larger than the top this can mean one of three things. One answer would be deformation of the breast. We know with Katy this is not true as her breast appear normal, attractive and proportionate to each other. The second reason would be because the breasts are real. C cups and larger natural breasts will naturally sag and cause the upper portion to be smaller than the bottom portion. Usually people who have had breast augmentation surgery will have top and bottom portions which are proportionate to each other. With this in mind, one final answer is available. When a woman already has C cup or larger breast and receives breast re-shaping surgery or a small breast implant the chest will look natural. This type of breast enhancement is less common and not as desired because most women who undergo breast implant surgery wants “perfect” looking breasts.

tl;dr - When the top of breasts are not as proportional as the bottom portion, the boobs are likely to be real.

Which means that we can conclude:

Kate Upton's breasts are real.

But I'll do some more research to confirm. For science.

How big are Kate Upton's boobs?

Gawker dares ask the question...

When did Kate Upton's boobs get their start? Esquire had a thoughtful interview with the caretaker of Kate Upton's Boobs, Kate Upton, who had this to say about her boobs's early days: "[I] do know that when I was in junior high, I used to be made fun of - for being flat-chested. Everyone would go, "She's not pretty! She doesn't have boobs!" So I always had boob envy. And when finally I went through my growth spurt, and they appeared, and I just... I loved them."

So how big are Kate Upton's boobs? They're quite big, capable of filling out most sweaters. This handy site reveals they are this big: 33DD [site taken down]

tl;dr 33DD

According to Funny Or Die, Kate Upton's breasts have the following talents: 

They can sing opera flawlessly
They can feed entire villages of impoverished, 
    3rd World Children
They can tell who’s the guilty one after only 10 minutes 
    of a Law & Order episode
They can eat more hot dogs than Kobayashi
They taught David Blaine everything he knows
They are skilled pointillism painters
They can complete high level Sudoku puzzles
They help Nic Cage bury his treasure on the weekends
They can juggle chainsaws

In other words, not only are Kate Upton's boobs real, but they have special powers!

And they've been the SI cover two years in a row.

Let's look at some more evidence:

Kate Upton boobs -- more data points. FOR SCIENCE.

Kate Upton before and after:

"Kate attended the religious Holy Trinity Episcopal Academy in Melbourne, Florida and is described as a ‘flat chested nerd’ by some former classmates... The good news for Kate is that she eventually blossomed and is now a very popular model. And by the way, she is all real and hasn’t had any plastic surgery."

And if there's any doubt, look how her breasts hang:

This picture might be photoshopped and airbrushed, but we reach the same conclusion.

Kate Upton's breasts are REAL.

Does Kate Upton have a boyfriend?

The answer: http://pandawhale.com/post/16803/kate-upton-boyfriend-nope

Best Kate Upton gifs: http://pandawhale.com/post/8490/the-best-kate-upton-gifs

Kate Upton in zero gravity:

http://pandawhale.com/post/37809/kate-upton-in-zero-g-sports-illustrated-swimsuit-2014-sicom

And, for something different, is Jen Selter's bum real:

http://pandawhale.com/post/41340/is-jen-selters-bum-real