I think the key issue here is that your vet, who has seen her and knows how she has been developing, has some concerns.
The guidance on bags of food is generally only a basic guideline and some dogs would need more, some less. A more typical problem though would be the opposite -- people giving too much, according to what the bag says! Almost always, the amount is in excess of what most dogs would need (this is really true of treats in particular -- they will say a dog the size of a cavalier can have for example 4-6 small milkbone/Bonio style treats and for most cavaliers, this would be nearly an entire day's calorie intake, in treats alone
).
Most puppies about her age would be eating about a cup of food daily. Puppies at that age can also get lanky; some are thinner and some are more plump.
I would ring back your vet and ask for more detail about their concerns. For one, most vets would not wish to spay or neuter a dog that is underweight. If they expressed a worry about this yet did the neuter, I would want to understand why -- that would indicate they felt the weight issue was pretty minor.
If the issue is simply that they think she isn't eating enough, I would certainly try increasing her food. Puppies this age will almost always eat what they need and she may not be getting enough food, particularly if she is active. All foods are different too. Doid they recommend feeding more? I'd increase by another half a cup daily.
It is always pretty difficult to make a judgement based on a picture.
Unless a dog is severely underweight, it is hard to see meaningful detail that a vet can see and feel. A more important view is actually looking down from above when the dog is standing -- to see how defined a waist they have. Also -- how prominent ribs are as well as spine and hipbones. These are things generally better felt, as a dog would be emaciated if these were visible, but a vet can feel whether a dog is underweight.
But going back to the starting point -- as your vet -- who would have seen your pup, felt her, and known her as she has developed -- feels concern, and you are understandably concerned about what they said, I would phone the office, and talk through your worries.