I tested adding a hidden field to a form using the Growth theme and it was completely hidden. This suggests that the issue you're experiencing could be dependent on the theme or form structure you're using.
In any case, you should be able to add a display: none css rule to the parent element if it's creating space around your hidden form element. Feel free to post the form HTML structure here and I can create a css path to hide the form column div for you.
I tested adding a hidden field to a form using the Growth theme and it was completely hidden. This suggests that the issue you're experiencing could be dependent on the theme or form structure you're using.
In any case, you should be able to add a display: none css rule to the parent element if it's creating space around your hidden form element. Feel free to post the form HTML structure here and I can create a css path to hide the form column div for you.