OK, say we have creatures A and B. They are any creature in the game that have different BODY_SIZE. Body shape/parts doesn't matter. Each has the properties unit.body.physical_attr_tissues[X], abbreviated as A0, A1, ..., B0, B1, etc.
A0/B0 ~= BodySize(A)/BodySize(B)
A1/B1 ~= BodySize(A)/BodySize(B)
A2/B2 ~= BodySize(A)/BodySize(B)^(2/3)
A3/B3 ~= BodySize(A)/BodySize(B)^(2/3)
A4/B4 ~= BodySize(A)/BodySize(B)^(1/3)
A5/B5 ~= BodySize(A)/BodySize(B)^(1/3)
0 and 1 are identical for inorganic creatures.
2 and 3 are identical for inorganic creatures.
4 and 5 are identical for inorganic creatures.
When the three pairs are not identical (ie. organic creatures), the difference is somehow related to strength and perhaps other variables as well.
When a dwarf was given 5000 strength via RAW file:
A0/A1=2.041~=2
A2/A3=1.608~=2^(2/3)
A4/A5=1.271~=2^(1/3)
When a dwarf was given 0 strength via RAW file:
A0/A1=0.7915
A2/A3=0.857=0.7915^(2/3)
A4/A5=0.925=0.7915^(1/3)
Considering that BODY_SIZE is a volume unit, this looks like some kind of Square-cube law data. 0 and 1 are volume related, 2 and 3 are area related, 4 and 5 are length related. The difference between 0 and 1 are something like "The creature is actually this big, but has the strength and weight of something this big". Inorganic creatures don't have muscle tissue with [THICKENS_ON_STRENGTH].
EDIT:
If I disable [THICKENS_ON_ENERGY_STORAGE] and [THICKENS_ON_STRENGTH] in tissue_template_default.txt, then organic creatures suddenly have identical (0,1), (2,3), (4,5) pairs when created in the arena.
unit.body.physical_attr_tissues[0] is creature volume corrected for thickness of muscle and fat tissue.
unit.body.physical_attr_tissues[1] is base creature volume (adjusted from racial volume by appearance modifiers)
unit.body.physical_attr_tissues[2] is corrected area?
unit.body.physical_attr_tissues[3] is base area?
unit.body.physical_attr_tissues[4] is corrected length/height?
unit.body.physical_attr_tissues[5] is base length/height?
I haven't yet found any effects of changing properties 2-5.