I'd suggest that how the text is displayed will depend on a lot of factors, such as: font(s) used; font(s) installed; any font substitution; text as Plain or RTF: any In-line embedded attachments; etc. plus the settings used by the email client user.
In your example, the receiver may have their setting to automatically enlarge the font, so when they send a reply, you receive your original text in the increased size.
If the Ariel font hadn't been used, which is a pretty good standard font that most users have installed, then you would probably have received the reply with your original text in a different font and maybe size.
So I guess the best way to keep the viewing as close to what you want is to stick with standard fonts and formats, just like web pages - and as you know they can appear different on different computers or with different browsers.
If you want more info, try some web searches on something like 'sending emails for consistent viewing' etc.