Meaning of "I'm so excited I could crush a grape"

It came from the children's tv show Crackerjack, in which a man called Stu Francis used to shout catchphrases,:

Stu Francis also did a stand-up routine on occasion, using such catchphrases as "Ooh I Could Crush A Grape/Jump Off A Doll`s House" etc.

also

Stu Francis (born 1951, Bolton, Lancashire, England) is a British comedian with a camp style of delivery who achieved celebrity as lead presenter on the children's television programme Crackerjack (1979–1984), on which his catchphrase was "Ooh! I could crush a grape".

This show was really popular, so "I'm so excited I could crush a grape." became in common use, basically meaning "I'm really excited", and the rest was just for comedy.

N.B., other catchphrases from Crackerjack include:

“Ooh, I could crush a grape/rip a tissue/pummel a peach”),

There seems to be some debate as to whether 'I could crush a grape' was used to mean 'I'm genuinely excited' or 'I'm supposed to be excited about this but I'm underwhelmed'. As it was used in a children's show watched also by adults, I would suggest that the children would assume the former but the real meaning for the adults was the latter. For the background see http://www.hulldailymail.co.uk/don-t-mind-remembered-crush-grape-man/story-11949409-detail/story.html