Thanks for the CnC, i can defiantly see what you're talking about with the neck so i will fix that. And for the arm i think it looks alright overall as the charater himself is very skinny i don't think that his upper arm would be that much bigger, or so i have learned on male anatomy.
Also on the girl, get right arm looks a bit thinker then usual. Make the line of her arm closer to her body so it does not look that big.
Thinking is the Enemy of Creativity. It's Self-Conscious, & Anything Self-Conscious is Lousy.
Everyone pointed out the anatomy issues already. I am seeing that you should practice with drawing straight lines, make a two dots from point A to point B and then put your pencil on point A look at point B and just draw while looking at point B. Takes some practice but in the end, you will learn straight lines. I bring this up because of your clothing, mainly the pants/shorts, have messy lines.
You should practice with fitting the clothing to the anatomy of the character. The guys left hand in his pocket seems flat, his hips/legs seem boxy. Females tend to have wider hips, she doesn't have wide hips, in fact, they seem too small.
I do like this piece, better anime style than I can draw. If you keep this up, you could possibly start learning how to make webcomics (or manga).