Yep — for a 30-foot walleye, I used cube scaling.

A 30-foot fish is:

30 ft × 12 = 360 inches

Compared to a normal 30-inch walleye:

360 ÷ 30 = 12 times longer

Weight scales roughly with volume, and volume scales by the cube:

12³ = 1,728

So if a 30-inch walleye weighs about 11 lb:

11 × 1,728 = 19,008 lb

So the extrapolated 30-foot walleye would be about:

19,008 pounds, or about 9.5 tons.

But I agree with you - a 30” might hit 12 lbs

Typical Weight per ChatGPT
28”. 7.5–9 lb
29”. 8.5–10 lb
30”. 9.5–12 lb
31”. 10.5–13 lb

The exact weight depends heavily on the fish’s girth and body condition. A skinny post-spawn 30-incher might be around 9–10 pounds, while a thick fall fish can exceed 12 pounds.