But can you explain me why the different fields in the form have different sizes?
This will depend on the theme you're using. Luckily, you can adjust each element with CSS. For example, if you wanted all fields to have the same width:
#post-66 .contact-form textarea,
#post-66 .contact-form input[type='text'],
#post-66 .contact-form input[type='email'] {
width: 100%;
}