"Concern" wouldn't be a word that I'd choose, but I'd be cautious. Set up an intake appointment with the vet of your choice and follow the guidelines that he/she gives you. Your pup is very young and the weight will be low. Within a litter you'll always see size difference, some due to sex, some are just smaller, there will always be a runt. I didn't get my girl until she was three months old and she was the last one to go (they kept a girl and a boy), she was the runt for sure, but at three months she had some weight to her. Get your pup on a good dog food. If you need assistance, go to the Dog Food Advisor for info. Large dogs have different needs than smaller dogs and there is a correlation between the food and whether they will have certain joint issues. Most of that site is open for review, only one part is a paid area.
https://www.dogfoodadvisor.com
I am so glad, personally that I used their advice. And being in Canada I have access to some products with higher ratings.
So glad that you took the time to join with us. You'll get great advice here, and you'll need it. Life with a Mastiff will always have questions. Oh, and we will want to see lots of pictures. Just love those puppy pics. Add as many as you like. We'll drool over them. Welcome!