The song was met with mixed reviews from music critics. The song peaked at number 9 in the United Kingdom (his third top 10 hit in that country), and reached the top ten in Ireland, Finland, Austria, Belgium and the Netherlands.
Many people speculate that Mika wrote the song as a tribute to Queen's "Fat Bottomed Girls", but in promotional materials for Life In Cartoon Motion, he states that this is not the case at all:
"I was flying to Los Angeles the next day and I can never sleep because I hate flying so much. So I was watching trashy television, it was two o'clock in the morning, a Victoria Wood documentary on Channel 4. It was about fat people in the United States and she visited a club called The Butterfly Lounge, which was the first place of its kind, a club for larger women to hang out in. Skinny women were not being allowed in. The women were amazing and I absolutely felt as if I had to write about them. I muted the television and wrote it straight away. I never expected it on the album, but a few weeks later we recorded it and it's now there. So it is one of my favourite tracks and brilliant to play live. Everyone sings along!"
The song features Afro-pop-inspired guitars and harmonies.
The song received mostly mixed reviews from music critics. Beth Johnson from Entertainment Weekly referred to the song as "an update of Queen's 'Fat Bottomed Girls'." Sal Cinquemani from Slant Magazine agreed, writing that the song " is nothing if not a disco-fied exaltation to Queen's 'Fat Bottomed Girls'." John Murphy from musicOMH wrote a negative review, saying that the song is "an ode to the delights of the larger lady, wraps up its laudable message inside a tune that grates in the worst possible way." Graham Griffith from About.com wrote that "the disco-lite 'Big Girl (You Are Beautiful),' which includes an admirable sentiment, fails to distinguish itself otherwise." Liz Colville from Pitchfork Media said that it is "a pumping, chorus-infested jam session complete with gospel backup singers that tragically feels the need to reassure big girls they can be beautiful."
Lizzie Ennever from BBC Music wrote a positive review, saying that the track is "upbeat, poppy, all-singing, all-dancing, big-band." Alexander Berntsen from Sputnikmusic wrote a positive review, saying that the song is "an incredibly fun (and happy!) tune, a genius moment. Hilarious indeed."
Mika has also rerecorded the track as the promo theme for the second season of the American comedy-drama series Ugly Betty, but with altered lyrics to say "Hey Betty, you are beautiful" in its chorus. Mika explained to the Sun newspaper: "Betty is a real fashion icon and we should celebrate anyone like her - genuine individuals who stay true to their own identity. That's what 'Big Girl' is all about so it was the perfect fit to change the song for her. It was a huge honor to be asked to do it. I love the show." In Australia, Crazy John TV advertisements used a sample of this song. In Spain, the Galician TV station Televisión de Galicia used samples of this song in the show O show dos Tonechos. The song is featured in the Ubisoft video games Just Dance 2 for the Nintendo Wii and Just Dance Now (with a remake).