Hey if you didn't read it, how would you know?
It's really hard to say without a scale, because (for example) my first chin came from a petstore and was HUGE! Nothing like what a good-sized 7-8 week old should weigh (surely he was much older)... and so when we first had a baby born here, I thought man is it tiny... even when it was 8 weeks old I thought it was tiny... but it was a healthy weight for that age.
I personally keep the babies longer than 7 weeks, we typically keep til 8 weeks and then separate them from mom for a week to check they're eating on their own and all, and then assuming they are, we either keep them or sell them or whatever... but some of it is dependent on weight as well.... I ideally want the babies to weigh 250+ by the 9 week mark, but I will let them leave if they are over 200 and steadily gaining... but without a scale, describing a chin that weighs 200 grams isn't all that easy to do...
If you think you should keep them longer, then keep them longer. It won't do any harm, but if they need more time with mom, it can do them good.
ETA: I'm not sure how much this would help, but... do you have a picture?