Where what and especially how are they copied?
– Is it a text template, an HTML message, or an Outlook graphic message?
– Is it on msWindows or in Unix/linux/chromebook/Android/Apple environment?
– Did you select the message or text in the message?
– Are there maybe things running on the remote desktop or other things reading and/or writing the copy and paste buffer underwater?

Just the basics: msWindows, Thunderbird, Outlook and copy text/images from Thuderbird to Outlook. Thunderbird then offers a type of HTML where you can set the text size to a number and to “large/normal/small” (or something like that) and the message can also use styles.
Then paste side into Outlook. There you have the option to set different methods through the Right Click Menu. Especially if there are tables and the like in the buffer. With the rightmost option to move text only.

There are a few more steps between Outlook and Gmail: Outlook is designed to be used with an Exchange Server environment. Microsoft defines the protocol between Outlook and Exchange. From exchange to Internet mail, it is the standard to use, although there may be many variations and options, including new developments that may not have been implemented yet. Add to this the fact that Google is also going its own way with Gmail and has its own developments. If you want to use Outlook to run Gmail, you have to set both to the right standard, but Microsoft and Google are not really friends in this regard. In my opinion, they each want to move forward with their own additions and not implement each other’s “standards” quickly.

Fortunately, we have ThunderBird as a lubricant. I’ve been using Google Mail with this since the beginning and it still works. I only use Outlook for Exchange, although I’m increasingly using Outloo-web-access there as well.