Email when it was originally invented was purely for ASCII text characters. There was no support for bold, italic, underline, foreign languages, multiple fonts, multiple font sizes, and certainly no in-line viewing of images.
Over time there have been various enhancements to bodge the addition of new formatting features but frankly the way each different email client handles this is typically different. These days bold, italic, underline, multiple font sizes works reasonably well, but multiple different fonts is starting to stray in to a troublesome area because you have no control over whether the other person has the same font(s) installed. In-line images is a whole new area of problems.
If the email was formatted as proper HTML then in theory the image position and size should be reasonably closely matched at both ends, however if you merely 'attache' an image file to an ordinary email then it is not a fully HTML formatted email, it is rather an email with a graphics image file attached to it which some email clients will try and display in the email for you without you have to save the attachment and open it.
In this case I believe you do have an email with an attached file which happens to be a graphics file.
If the recipient 'saves' this attachment and opens it then I would expect it to open in a graphics program at the full original size.
As a reminder a fully HTML formatted email can have graphics which are 'attached' to the email, but another way to do it is to have HTML commands which load the image from a web-server without them having to be attached to the email. I find this later approach which not only results in much smaller emails is less likely to result in formatting problems because it is much more like a real web-page.
By the way, another test you could do would be to forward the email back you you with the attachment.