media_removed_toofab 2013

Kim Kardashian
isn't the only one in the family changing in size ... but Rob Kardashian doesn't have a pregnancy to blame.

On this week's episode of "Keeping Up with the Kardashians," Rob admitted he gained 40 pounds while dating Rita Ora.

He definitely looked noticeably heavier when cameras snapped him outside of an L.A. office building earlier this week (right), especially when compared to how fit he was on "Dancing with the Stars" back in 2011 (left).

But Rob is on a mission to drop the weight.

"This is what fat boy now has to eat for breakfast today. So much fun! Lol. Cardio time," he posted yesterday on Twitter ... along with a shot of an healthy looking meal.

"Another fat boy breakfast. Let's go! 5 mile run next. 50 pounds gots to go," he added this morning.

Good luck, Rob! Can't wait to see his progress.

Check out more stars who have fought the battle of the bulge and won in our Celebrity Weight Loss gallery above!

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