Alternative image sizes are too large

Very new to WordPress here so still learning a lot so if anything I say here is silly then please go easy on me

On a site I've created I'm using this plugin here :

https://wordpress.org/plugins/simple-image-sizes/

I believe I'm hopefully correct in saying that this is basically allowing me to create smaller images from the main uploaded image in exactly the same way it would as defining the sizes in the functions.php file?

I'm not one much for working without a graphical interface so prefer this way of working wherever possible.

Anyway I'm not sure if I'm making some sort of rookie mistake as I've made an image size which is smaller than the uploaded image and where the image is being used (currently being output by a Beaver Builder module) it is indeed being spat out at the correct pixel size however it is a very large image, around 450kb whereby if I download the image to my computer and run it through Image Optim (OSX) then it comes down to around 65kb which is waaaaayyy better as you can imagine. I've got about 6 of these images on one page and so as you can see the page size is very very bloated.

Is there something I've missed as to why WordPress is only resizing the images but it's not compressing them or is everything doing its job correctly and there's something else I need to do / install in order to compress images too?

I thought I'd read that WordPress did compress images but I may have misread that somewhere?

Thanks in advance for any help with this.

Best wishes,

Mark