The analysis of the results below uses only the data couples (weight + length). The weight-only are not used.The point cloud below allows us tu process the theoric chart of te weight/size ratio for the species of the studbook.
Process of the theoric chartData are adjusted with a 3rd degree polynom, i.e. it is such as y = a + bx + cx² + dx
3We use the mathematic formulas of the least squares to get 4 equations with 4 unklnowns a, b, c et d :

Thus :6840 = 7
a + 329.7
b + 63898.29
c + 15652729.383
d535608 = 329.7
a + 63898.29
b + 15652729.383
c + 3906837663.4641
d114120141.6 = 63898.29
a + 15652729.383
b + 3906837663.4641
c + 976575536200.69
d28157819518.32 = 15652729.383
a + 3906837663.4641
b + 976575536200.69
c + 2.4414092413413E+14
dWith the Gauss pivot we get :6840 = 7
a + 329.7
b + 63898.29
c + 15652729.383
d213444 = 0
a + 48369.42
b + 12643119.924
c + -724600434.0153
d51682383.942857 = 0
a + 12643119.924
b + 3323553168.4749
c + 833692301714.06
d12862866806.931 = 0
a + 3169594109.5248
b + 833692301714.06
c + 2.0913979025735E+14
dthen :
6840 = 7
a + 329.7
b + 63898.29
c + 15652729.383
d213444 = 0
a + 48369.42
b + 12643119.924
c + -724600434.0153
d-4109024.9485097 = 0
a + 0
b + 18810597.390862
c + 1023093170780.5
d-1123880297.1978 = 0
a + 0
b + 5202763548.7037
c + 2.5662204800231E+14
dthen :
6840 = 7
a + 329.7
b + 63898.29
c + 15652729.383
d213444 = 0
a + 48369.42
b + 12643119.924
c + -724600434.0153
d-4109024.9485097 = 0
a + 0
b + 18810597.390862
c + 1023093170780.5
d12621897.740723 = 0
a + 0
b + 0
c + -26352051398170
dThus :d = -4.7897211302494E-07b = 54.694079564966c = -0.19239111738352a = 158.33180004204We get the chart below (in red) :y = 158.33 + 54.69 x + -0.19 x² + -0.00 x
3simplified by (in green) :y = 54.69 x + -0.19 x²
Bodymass index (BMI) of all the Malaclemmys terrapin
BMI = Weight (g) / size² (cm.mm)
Histogram of the distribution of the BMI of all the Malaclemmys terrapin
Histogram of the distribution of the BMI of all the Malaclemmys terrapin juveniles
Histogram of the distribution of the BMI of all the Malaclemmys terrapin adults

The Bodymass index (BMI) is "the weight divided by the square of the size". Do it for yourself with your size in m,cm and your weight in kg, You will get a value between 20 and 25. A value above is said an overweight (and the same goes below 20). It is a good index, ask your doctor.
Mean of the BMI of the Malaclemmys terrapin : 5.97Standard Deviation of the BMI of the Malaclemmys terrapin : 4.54