The 26-year-old recently revealed that he was on a quest to lose 50lbs, after gaining the weight following his split from pop singer Rita Ora last December.
Speaking with Ryan Seacrest on the Ryan Seacrest With the Kardashians: E! News Presents special on Sunday night, Robert admitted that he is self conscious about his body image.
'I cry because my penis looks so small now'
'Every time I hop in the shower and see myself naked I cry, because my penis looks so small now,' he said.
Robert urged his sister Kim to move a plate of French fries away from him before adding: 'The upside is, every time you lose 10 lbs you gain an inch on your penis.'
Despite his light hearted response, sister Khloe assured the American Idol host that loosing weight has been tough for her younger brother.
The Keeping Up With The Kardashians star recently spoke to a US magazine about his diet and exercise program.
'I lost 5 to 7 [pounds] last week. I'm trying to lose 40. It's a little journey, but it's good... I'm like 240lbs right now, probably 235lbs,' he said.
Speaking about his exercise regime, the reality star revealed that as well as having a personal trainer, he has also enlisted the help of his former Dancing With the Stars partner Cheryl Burke.
‘We’ve started running and doing our own little workout, it’s good. 'I'll [exercise with] my own trainer at eight in the morning before all that,’ he said.
In February Robert documented his weight loss efforts on Twitter, posting a series of photos of healthy meals he had been preparing for himself for breakfast, lunch and dinner.
‘This is what fat boy now has to eat for breakfast today. So much fun! Lol. Cardio time!,' he tweeted in one of his posts.