Have you ever been in the situation where you have uploaded a large image, for example 700px width, however it still appears to be very small when inserted in the post? If yes, most probably because you overlook the option to select image size before inserting into post.
By default, WordPress will select a Medium image size which is around 300px width. That’s the reason why you’ll see small image in the post, although you’ve uploaded a large image.
Be sure to to select the size choices (located on the bottom right), either Large or Full Size. Refer to screencast below: