The labels in your choices are too long to be positioned in the side-by-side layout. We don't currently allow you to control over the amount of horizontal space allocated for a choice label. The current behavior is that a choice grows to be as wide as its label, with the width of the form being the only limiting factor.
The default width for all of the choices in a Single/Multiple selection field is the width of you longest choice label (as you have seen). In order for you to get side by side you will need to make your form wider and then make the multiple selection field wider.
If these steps do not work then the next step would be to use a font size smaller and/or change font to Times New Roman (which is narrower than other fonts).