Spacing in mathastext package

I have this MWE, using mathastext package.

documentclass[12pt]{article}
usepackage[T1]{fontenc}
usepackage{mathtools}

usepackage{newtxtext}
usepackage[italic,defaultmathsizes,symbolgreek]{mathastext}

begin{document}
begin{equation}
sum_{k=1}^{infty}int^{n-1}_{2n+4n}{f_n(k,n),dx}
end{equation}

begin{equation}
sum_{k=1}^{infty}(-1)^k sin k
end{equation}

begin{equation}
3n+1=6n-sum_{substack{0 le i le m  0 leq j leq n}} P(i, j) 
end{equation}

end{document}

Is it usual that this package have closer spacing? See the red rectangles.