Theme Horse

Hi bk,
This attachement page display image in thumbnail view. Its wordpress default feature. If you like to do it with css then you may add this below css in your custom css

img.attachment-medium{
  width:100%;
}

Other wise it needs code customization and you need to hire a developer to fix it. View below link
https://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src

Thank you!