When I preview an email using Hubspot's own Preview feature from the editor - the font size appears as expected. As in, the font size persists from desktop to mobile (or tablet) view and the only difference is that the width/height adapt to the devices' screen dimensions.
However, when I actually send a test email to myself, the font is tiny when I open it via my Gmail mobile app. Others in my organization report seeing this as well on their phones. When viewing the test email via desktop, it looks fine. Not sure why the font size seems to shrink arbitrarily on mobile.
Based on your email I would recommend increasing the font size. For mobile devices you should not be using anything lower than 16 px for most readable text. In addition, the contrast ratio of your text to background fails because for the font size the foreground (146, 146, 146) is only a 3.11:1 ratio to the white background and should be closer to a 4.5:1 ratio. I would increase your font size of the body text to at least 16 px and increase your header slightly to keep your ratio and increase that to 17 or 18px. Try that and see what happens.
Can you post the preview link or the view in browser link from an actual email for us to evaluate the code? Unfortunately without code we would only be guessing at potential solutions.
Based on your email I would recommend increasing the font size. For mobile devices you should not be using anything lower than 16 px for most readable text. In addition, the contrast ratio of your text to background fails because for the font size the foreground (146, 146, 146) is only a 3.11:1 ratio to the white background and should be closer to a 4.5:1 ratio. I would increase your font size of the body text to at least 16 px and increase your header slightly to keep your ratio and increase that to 17 or 18px. Try that and see what happens.