You can't compare any drugs by their dosage as there's a lot of...

You can't compare any drugs by their dosage as there's a lot of factors that come into play and they won't be equivalent.

The molecular structure can affect the weight so 100mg of one drug might contain say 100 'drug pieces' while a smaller drug might have 400 'drug pieces' in that same 100mg

Also the way that the body reacts to the drug comes into play as well. For instance in the example above if 100% of my 100 drug pieces bind to where I want it to them that's great but if it binds at 25% then I'll need to increase my dose to 400 drug pieces to get 100 pieces to bind so I'll need a dose of 400mg.

There's also a bunch of other stuff like how the drug might be eliminated from your body where it might be quicker or slower but we'll suffice to say there's a lot of factors at play