I think what's important here is if you're comfortable & confident wearing a maxi. IMO the m/l flap is more proportional size wise but that doesn't mean that you can't pull off the jumbo/maxi. If you're not usually a big bag kind of girl or have hesitations, maybe opt for the jumbo instead. Check out the celebrities & the in action threads for pics of petites making it look effortless.
I'm 5'5" and 130 lbs so not petite but here is a pic of me with my maxi. I debated over the jumbo and the maxi but got the maxi because I liked the length of the strap. The Jumbo was awkward on me. Anyways, hope this helps!