I think it might be difficult to translate, before it's a form of expression.
少しは... 片眉分くらいはこいつも重さを感じるんじゃないかって
Which Google translates to:
A little ... I guess this guy feels weight as much as one eyebrow
Probably meaning something along the lines that Reimu isn't being weighed down, more than the weight of a single eyebrow, compared to the speculations Marisa is having, making her a bit jealous of how carefree Reimu is.
