Looking at it, I think @Strontium hit the nail on the head. I've had this happen too when I work digitally -- if I work too big, I tend to make my details too fine, and those small details get too indistinct when they're seen at the final size.
If you think about it, it makes sense -- if you shrink a 2-pixel-wide line to half the size, there's no way to get a super crisp line, because it can only be one pixel wide with some antialiasing to smooth it out. You probably don't see the same effects in the page I linked just because my lineweight is VERY THICK in the print-size version -- If you look at smaller text on my page, though, you'll see the same effect as it gets smaller, just by virtue of having fewer pixels to work with.
If you want to get the same effect your pages have at-size, I wonder if it'd be helpful to actually like... draw a page at 900px wide, like you're used to, and then, as a test, scale that picture up to 3000px or whatever size you'd print at, and then look at how thick the lines are at that size, and how big your smallest details are at that size? So then when you're working at 3000px, you'd know how thick the lines need to be in order to shrink nicely. (Though I'm realising now that maybe this is what you mean by scaling the same ratios with larger images, in which case, I am sorry for explaining the thing you just said!! xD )