A blog receiving 20,000 daily visitors consume bandwidth based on the size of images you use on your blog, the template, Content Management System, JavaScript code and the size of your posts.
If your blog does not have images, it has a clean code and receives 10,000 daily visitors, you are supposed to utilize 100MBs to 1GB of bandwidth per day, it it exceeds that maximum bandwidth usage, make sure you consult further.
If your blog uses images, then it must utilize between 800MBs to 3GB of bandwidth—if it exceeds this limit, then you are required to compress your images and ensure the JavaScript code is clean.
Many popular bloggers use CloudFlare for protection against malware and also for bandwidth reduction. This company has a free offer for people who want to test their products.
It is advisable to use images not exceeding 50KB for any post your update, unless it’s an image you cannot compress; you can use snipping tool or Photoshop to compress your images, or WordPress plugins.