Possibly you are an artist or a graphic designer and you have created a beautiful presentation template using shapes, images and your great talent. Like these Indonesian people from that site.
All templatess look very good. Really, I much like it. But if you'll download for example that template, then (after unzipping) you'll see that its size is 16 mb! And it's only a presentation template! Without any your data, images, tables or charts! It's too much anyway.
Special for creative people LibreOffice Impress can offer a mechanism for presentation size decreasng! Just use a main menu item "Tools-Minimize presentation". It opens a wizard that will help you decrease your presentation size.
Here you can select a general size setting in DPI for your presentation. 150 DPI is a good choice for many cases. Press "Next" button.
I wouldn't change default settings on that page. Press "Next" button.
Here you can select some options for images in your presentation. Lossless compression is choice for saving you images without any compression. I always select JPEG compression there with 75% quality and 150 DPI resolution. I check the both options: "Delete cropped image areas" and "Embed external images". Press "Next" button.
We see options for OLE object replacing. It means all OLE objects in your presentation, like shapes, spreadsheets, text fragments, etc. will be replace with its images. Press "Next" button.
You can see it some summary. Look at number of images in our template! 259 images! Always select a "Duplicate presentation..." option to save your changed presentation into another file and save an original file as is. Press "Finish" button.
Find some place for saving your small presentation and press "Save" button in "Save as" dialog.
I got only 0,9 mb instead 16 mb for our example! You'll can get the same great result for your great and beautiful Impress presentations.