Try looking at the source image, use f12 and inspect the element, that should give you the image. If the image is not final, you could save as that image to your application stack and override the image with a bigger icon. If it is final, then the really problem starts in, you need to override the function, and that would actually lead to more complications, and would have impacts after up-gradation to a higher version of Pega, if Pega changes those functions in higher version.
Oh, I see what you mean! Is there a css class associated with those labels that has + and - if they do you can override them using the css overrides and increase their font-size. probably a good Idea to paste the generated HTML of that + and - and the rules involved, I might be able to help.