Why are you doing that? Just create the table in Muse.
That's an odd workflow for getting tables into web pages. More accurately, you're putting pictures of tables in web pages. Lack of native tables in Muse is one of its key weaknesses.
Assuming you believe there are good reasons for doing it your way, maybe you could streamline it a bit, and solve some problems.
First off, there's no reason to export to PDF. Just select the text frame which contains the table and copy it. Paste in Illustrator.
In Illustrator, if you're not already working with pixels as the unit of measure, switch to that. Rt-click the ruler, (View > Rulers > Show Rulers), and choose Pixels.
At the bottom of the Transform panel, (Window > Transform), tick Align to Pixel Grid.
Now, you say your tables are of a set size, and you're losing that size by the time you import to Muse. I don't presently run Muse, although I did experiment heavily with an early version. In any case, I can't test any particular method right now, but it stands to reason that you want to exert all possible control over size as you Save for Web from Illustrator. So, hopefully you know the pixel dimensions of the space your table will occupy on the Muse page. Set your Illustrator Artboard size to those dimensions, (dbl-click the Artboard tool), position your table in that space as desired, and Save for Web. In the Image Size panel of the Illustrator Save for Web dialog, (something else I don't regularly use), tick Clip to Artboard and confirm the desired dimensions are shown.
If, for some reason, the resulting import to Muse is again undersized, don't upsize it there. Check its size and determine mathematically how much smaller it is. That way, you can redo the Save for Web from Illustrator, adding a size adjustment to compensate for the loss. It may take a bit of trial and error to get the right adjustment, but the result will be better than you're getting when manually resizing in Muse. Ideally, you don't want to resize any imported graphic in Muse.
Ah, no tables.
But there's plenty of ways of doing it with plain text which is still
far better than a picture of text.
No argument here, Bob.
There are some ways of getting live tables into a Muse page via Dreamweaver as well, but I made the assumption the OP's use of Muse belies an aversion to DW in the first place.