Does it matter? They both look ok from here

I would prefer them to be the same size but I cannot see the code or option is SD or Webform Designer that allows me to do that.

As Eric says
in order to advise on that with any conviction one would need to see both the forms project files and site designer project file, one couldn't even begin to guess just from an image, it could be one or the other or a combination of both

To Hazard a guess check the FORM WIDTH settings in Web Form Builder

To hazard another guess, if the form widths are the same, check margins & padding

Here is another idea

open the form you prefer in web form builder
save it as (new name, )

change input fields/labels as required
take away or add anything as required

check out each form in "preview' are they the same size??
if so
save the form as required (new name or replacing the old file, which will then override

publish the form to your S-Drive which will again override that on your S-Drive

1/ open the forms in browser

see if they are still the same size etc,,

if so link the new form in Site Designer same as you did previously

If not then it seems logical to assume that the problem therein lies within Site Designer.

Many thanks. Your method has helped me solve where the difference was - maximum width different in each form.