Heart rate vs body size
The paper you link finds that there is not a significant correlation between body mass and resting heart rate in humans. While there is a well known negative correlation across mammals:



This is the ever-popular mouse-to-elephant curve that is found in one form or another for lots of physiological measurements.

I'll suggest, but have no evidence, that the lack of a significant correlation in humans is simply a result of there not being enough range in body mass among humans to allow finding a significant result. When the range of one variable is small, the association between that and another variable necessarily lessens. For example:



Although the linear relationship between the two variables is largely similar, when the range of x is truncated, the coefficient of determination, $R^2$, decreases. The correlation goes from 0.97 to 0.7. In this case, that might still be "significant", but I suspect that in humans that variation is much greater in resting heart rate.

It would have been nice if that paper included the actual data on HR, but they act like it doesn't exist just because it was not significant.