The basic problem you're running into is this:
1. You're expecting damage to scale consistently.
2. However, damage for ST in GURPS does NOT scale consistently with ST, mostly to allow more variation at normal human range.
3. Therefore, if you want a consistent 5 to 1 scale for damage, you can't have dST scale on a straight 5 to 1 basis.
Is that a correct interpretation?
If that is the case, remember you would want Lifting ST and HP to scale separately from Striking ST.
Personally, I would just ignore the wonky damage progression and simply go with straight 5 to 1 or 10 to ratio. That's how it works for D-Scale, C-Scale, etc. damage. If large creatures seem too vulnerable, Pizard has an interesting house rule
here for impaling/piercing damage. Trying to make damage scale correctly is more trouble than it's worth. You'd be better off reworking damage progression.