the widest part of his body was about 3 times the size of the gaps of the cage, question i have is are they able to flatten their body enough to fit through the gaps?
Oh dear. I'm sorry about the budgies.
To answer your question, when I touch my stimmie she'll sometimes flatten her body at that point. I reckon she could easily fit through a gap that looks three times thinner than her body. I've also seen her squeeze through some very thin gaps during her wanderings. She seems to like going through small gaps. She could go around, but no, she has to squeeze through them.