To add in, at least in the US it's a little less to do with the therapeutic range and a whole lot more with the safety range,and what was tested during clinical testing.

Since we often look at log differences in the clinic when it comes to safety, you can safely assume for most drugs prescribed at x mg/day that they will still be efficacious in a fairly large range of adult weight ranges.

However, especially with some drugs as you have indicated, this isn't the case. Personally I am often worried with something like hormonal birth control, which we are constantly trying to reduce the dose to limit side effects, however there is efficacy issues with larger patients.