Well that’s not as easy as you think, responsive html design as you see in the internet is pretty descriptive. Without any layout-information the rendering part doesnt know which column should take the empty space that’s there, or if an image that is part of the table should be scaled down to fit the column or extend the column, and if it’s get extended should text in another column rendered in multiple lines or should it stay in a single line… and so on…
The table renderer tries to render a table as best as possible ( it extends the with of the table to the width of the slide, which could cause the result that you described )
I whould suggest to wrap the table in an anonymous block and annotate the block with a fixed font size
::: block
First Header | Second Header
------------ | ------------
Content from cell 1 | Content from cell 2
Content in the first column | Content in the second column
::: <!-- element style="font-size:16px" -->