I have no experience going down that deep, but the Math says that that for an average person. You would need to dive to 165m with a full lung to get to the same volume of air in your lungs as FRC to 25m.
So I don't believe the problem is severely collapsed lungs. Sadly these formulas are not perfect for people as we are not perfect containers. FRC simulates collapsing of the lungs, but not the pressure. The pressure against your lung-tissue is lower with FRC because there is less pressure on you period.
At 25m FRC the lung is just the same size and flooded with blood at the same level as if you were at a 165m Full lung dive (assuming you are the average man).
I don't know the specifics, but I could imagine that there are tissues in the lung that are damaged by the pressure, not just by having blood flooding in. The other alternative is the blood is not coming from you lungs, but something further up the respiratory system such as your nose. Not sure what it takes for you to equalize at that kind of depth.
It has been more than a decade since I had a physics class so please feel free to fix any of my formulas (darn difficult to write in type-writer text).
Average Person: based on
wikipedia
Total Lung Capacity (TLC): 6.00 L
Vital Capacity (VC): 4.80 L
Residual Volume (RV): 1.20 L
Volume in the Lung (LV)
Depth (D)
Boyle's law: P1xV1 = P2xV2 or V2 = (P1xV1)/P2
Water Pressure at depth: P = 1atm + D * (1atm/ 10m)
So where V2 is Volume at Depth, V1 Volume at survace.
V2 = ( 1 atm*V1 ) / ( 1atm + (D * 1atm/10m) )
V2 = V1 / (1 + D/10m )
or in diving terms
LV = TLC / ( 1 + D/10m ) or RV / ( 1 + D/10m ) for FRC
So in the FRC case, assuming you "normal"
LV = 1.20l / ( 1 + 25m/10m) ) = 0.34l
In the case of your Full Lung attempt
LV = 6.00l / (1 + 70m/10m) = 0.75l
Calculate equivalent depths:
D = 10m * (V1/V2 - 1)
or Depth Full Lung (DFL) to match Depth FRC (DRV) is.
DFL = 10m * (TLC/(RV/(1 + DRV/10m)) - 1 )
DFL = 10m * ( (TLC * (1 +DRV/10m) )/RV - 1)
