I'm not sure there is a fix - this is a design decision. The problem is the table is set to a specific width and each column knows how wide it needs to be. Then you add another column and Word needs to make a decision: Does it resize the existing columns (which you were previously happy with) to make room for the new column or does it leave those columns untouched and add the new column to match the width of the source column. Effectively, the programmers will always make some people unhappy, whichever way they decide.

However, as I experiment with a new table in Word 2016 there is something changeable that influences behaviour of the table. I haven't worked it out yet but there is something going on that gives hope to finding the setting.

Inserting a new table, I can select a column and add a column and the total width doesn't change - the other columns shrink to accomodate the new column. However, if I drag the last column just a tiny bit wider than the margin, adding a new column gives you a much wider table going off the page. On the other hand, dragging the table smaller than the page margins allows me to add a column and other columns adjust to retain the same total width. Is the total table width the key?

Tweaking settings like Table Properties > Options > Automatically resize to fit contents may be the key but I'm not convinced of that.

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Andrew Lockton
Chrysalis Design, Melbourne Australia