These are some thoughts on disabling WordPress from creating multiple thumbnails, it might work for you and might not depends on the theme yo have, plugins etc …
– Turn off any plugins.
– Switch to your default theme.
– Set all image sizes to 0 in the Settings –> Media settings.
This should keep WP from generating those extra thumbnails sizes. WP should creates only one additional thumbnail around 20-25Kb
If this doesn’t work for you, then either a plugin or the theme you are using is making some changes.
Some themes have functions like
update_option('large_size_w', 500); in the
functions.php file, so you will need to check that file.
hope it helps