Thanks for sending an email! In taking a look at the site a bit, I believe I was able to come up with some code suggestions so that you can get the desired feel.
Just above the actual code for the signup form, it looks like there was some custom CSS added. A lot of what's set in this section is what's creating the design that's there now, so let's see if we can tweak it a bit to get what we're looking for.
First, let's adjust the email address input field's width. Find this chunk of code:
Change the width to a smaller percentage. It looks like the percentage is how your theme's managing the responsiveness, so if you make this width smaller, you should see that the input field gets smaller too and should remain fluid.
Next, let's change the subscribe button's width. Find this code here:
And decrease the width percentage as well. You should be able to emulate a similar design to the 1.2 version in doing the above. If in making these changes you're still seeing issues, feel free to post back here and I'd be happy to help!