style sheet bloats HTML

Need help with understanding and fixing a problem that I discovered recently on my site.
The size of the HTML code of all the pages increased dramatically from a previous version. The average size of the HTML code is now about 460 kbyte for each page but most of it is not content. If I look at the source code of any of my site pages, I find a very big style sheet that starts with:

<style type=”text/css” media=”all”>div.wpcf7{margin:0;padding:0}div.wpcf7-response-output{margin:2em 0.5em 1em;padding:0.2em 1em}div.wpcf7 .screen-reader-response{position:absolute;overflow:hidden;clip:rect(1px,1px,1px,1px);height:1px;width:1px;margin:0;padding:0;border:0}div.wpcf7-mail-sent-ok{border:2px solid #398f14}div.wpcf7-mail-sent-ng{border:2px solid #ff0000}div.wpcf7-spam-blocked{border:2px solid #ffa500}div.wpcf7-validation-errors{border:2px solid ………….
…..
I stop here as it is very very long.
You can directly check it by looking to the source code of my site “http://hr-assistant.it&#8221;. I posted on WordPress.org and I got the suggestion to contact you as the problem may be related to the Contact Form 7 plugin.

This code is not present in the HTML code generated by the development version of the site that I have on another server (that as well uses Contact Form 7). I am struggling to find why it is there and to understand if I can eliminate it from most of the pages (except the contact form) or have it be loaded from the server at load time.
If I deactivate the plugin the above code does not disappear.

Your help is highly appreciated.