Some Boxers can take 2-3 years to fully mature and body up.
Also some bloodlines are slower to mature too so just because one
is nice and 'chunky' doesn't mean to say the next one you meet in the street will be.
I wouldn't be worrying at this stage so much if she's just 'lean' however if her pin bones
(hip bones) are protruding on her back then I would be getting concerned.
The feeding amounts on the dog food bags are 'averages' they aren't stuck in stone.
Some dogs require more than the advised amount others less.
Some dogs can benefit from a change in completes. Others benefit from say tripe being added to the food.
Normally the 'old fashioned' way of feeding - meat and biscuit diet - they used to up and down the biscuit for weight loss/gain.
The meat content would always be the same.
How much exercise are you giving Nor?
As sometimes without realising it, some can be over doing the exercise and just need to reduce it.
The other thing is that if you are intending to show Nor, the Boxer dogs that are shown are on the leaner side.
If you are around towards the end of March, you can see the Essex and Eastern Boxer Club's champ show and you'll see
all the different types and weights throughout the classes.
Here's the club website
http://www.essexandeasterncountiesboxerclub.co.uk/