Here's a MWE that produces the oddly distributed columns:

\setuplayout[backspace=0.5in,width=4.5in,topspace=0.5in,top=0in,topdistance=0in,
  header=0in,headerdistance=0in,bottom=0.25in,bottomdistance=0.125in,
  footer=0.25in,footerdistance=0.25in,height=7.75in]
\setuppagenumbering[location={footer,middle}]
\setupwhitespace[quarterline]
\setupbodyfont[pagella,9pt]
\definepapersize[digest][width=5.5in,height=8.5in]
\setuppapersize[digest]
\starttext
\startplacetable[location=force,title={Stun Levels}]
\definecolor[back-3][r=.8,g=.8,b=.8]
\setupTABLE[background=color,frame=off,framecolor=white]
\setupTABLE[row][odd][backgroundcolor=back-3]
\setupTABLE[row][1][topframe=on,bottomframe=on,framecolor=black,rulethickness=0.75pt]
\setupTABLE[row][4][bottomframe=on,framecolor=black,rulethickness=0.75pt]
\bTABLE[width=broad]
\bTR\bTD {\bf Damage >\\Soak by:}\          \eTD\bTD {\bf Stun\\Level}\eTD\bTD {\bf Stun\\Effects}       \eTD\eTR
\bTR\bTD 0 or less\eTD\bTD Unharmed         \eTD\bTD None.                                               \eTD\eTR
\bTR\bTD 1 to 8   \eTD\bTD Stunned          \eTD\bTD -1D on all actions for 2D rounds.                   \eTD\eTR
\bTR\bTD 9+       \eTD\bTD Severely\\Stunned\eTD\bTD The character is knocked unconscious for 2D minutes.\eTD\eTR
\eTABLE
\stopplacetable
\stoptext

Naively, I expected the third column, which has the longest text, to absorb more of the width, rather than the three columns getting about the same width.

I added the following two lines before the \bTABLE:

\setupTABLE[column][1][width=6em]
\setupTABLE[column][2][width=6em]

This caused all the space not used by the first two columns to be used by the third column.

Thinking about it a little more, the first result makes sense: there is probably no way for ConTeXt to figure out that the third column should be wider unless it is given some constraints.

Interestingly, groff's tbl has a Column Specifier, x or X, that, after computing all the widths of columns without an x specifier, uses the remaining line width for the columns with an x, distributing them equally across the columns with x specifiers.  (I often use groff for quick-and-dirty documents, or where ConTeXt is not installed; I use ConTeXt for documents that are more complex, or ones that need their appearance customized.)  This is particularly useful when one column has paragraph length text contents, and the other columns don't.

I don't suppose there is a bTABLE equivalent?  Or an equivalent in one of ConTeXt's other table types? 

On Sun, Dec 6, 2020 at 11:20 PM Aditya Mahajan <adityam@umich.edu> wrote:
On Sun, 6 Dec 2020, T. Kurt Bond wrote:
> On Sun, Dec 6, 2020 at 2:20 AM Aditya Mahajan <adityam@umich.edu> wrote:
>
> > On Sun, 6 Dec 2020, T. Kurt Bond wrote:
> >
> > > Setting maxwidth and textwidth on the \bTABLE doesn't seem to do
> > anything.
> >
> > Use:
> >
> > \bTABLE[width=broad]
> >
> > \eTABLE

> That did the trick!  It did distribute the new width oddly, with adding
> more width to the the first and second columns, while the third column,
> which had the most text to wrap, was not as wide as it could have been.  I
> had to set widths for the first and second columns, and had to experiment
> to get them right.

A MWE (Minimal working example) is needed to see what is going on.

Aditya
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T. Kurt Bond, tkurtbond@gmail.com, https://tkurtbond.github.io