system
Given the following post, respond as if you're commenting on Reddit:
human
Why is medicine dosage linked so closely with weight?

I work in a pharmacy and specifically in children the pharmacists will constantly check the patients weight for dosage. I found myself wondering why the dosage is linked so closely with weight, instead of some other factor. When I asked the pharmacist I was working with he didn't seem to know exactly why. All he told me was that if the patients weight is higher they can take a higher dosage of the medicine. I did a google search but all I got were links on calculating dosages based on weight. 

I apologize if this is too broad a question. I just want to understand why weight is the highest determining factor in dosage for children.

Edit: thanks so much for all the answers. I think I understand a little better. Much appreciated.
gpt
More weight means more cells and more blood/vascular structure to feed all of those cells (regardless of whether this is weight from muscle, fat, or bone). Thus, with a heavier person with more cells and more blood would need a higher dosage to achieve the same blood concentration of the medication.