Yes, but it is still potentially useful to know how much of the heritability is metabolically vs. behaviorally manifested.

Also more generally, we should be careful about mixing different levels of causation.

I'm not too clear on how to interpret hierarchical model coefficients, but they do give at least one description of effect size, on pg6:

These associations revealed clinically meaningful differences in weight. For example, participants who scored in the top 10% of Neuroticism's Impulsiveness weighed, on average, over 11 Kg more than those who scored in the lowest 10% of this trait. Likewise, participants who scored high on Conscientiousness's Order weighed about 4.5 Kg less than those who scored low on Order.

and pg8:

In addition, the emotional aspects of impulsivity (N5: Impulsiveness and E5: Excitement-Seeking) were also associated with greater increases in adiposity. For example, on average, at age 30, those who scored one standard deviation above the mean on impulsivity had a BMI that was approximately 2.30 points higher than those who scored one standard deviation below the mean on this trait. By age 90, this gap increased to a 5.22 BMI point difference (see Figure 3).