The short answer is that with few exceptions you should NEVER need to scale down .mobi files because Amazon.com will do it for you.
I think 5.5 MB is a reasonable file size for an epub file. I don't know if it is a typo, but 60MB is a bit high for mobi files.
However, when you buy a .mobi file directly from the publisher, the preferred way to get it to your device is to upload it to Amazon's Personal Document Server which will feed it via wifi to your device.
For mobi files, I do NOT recommend trying to transfer it to your device via USB because it requires you to put a bloated file onto a device which may not be ready to process it.
Uploading it to Amazon.com first will let Amazon to send to device a file size appropriate for it.
I was testing an ebook before I published it on Amazon.com. The epub file had high quality graphics and was 7.6 MB. The Mobi file was 16mb, but after I uploaded it to the Kindle's Personal Document server, I think the Kindle e-ink device only needed to download a smaller 1.5 MB file size to the device. It looked perfect even at that smaller size. Amazon reduced the size and resolution of graphics to match the device.
(It's possible that O'Reilly has multimedia files which are causing the bloat, but i doubt it.)
Some authors and publishers have problems optimizing their graphics and file sizes, but the Oreilly guys producing those ebooks are the best in the industry. So they are providing a book optimized for several different platforms and devices.
If you ordered the ebook directly from Amazon, then Amazon would automatically download the file size appropriate for your device. But when you purchase the ebook directly from the publisher, you are buying essentially a file before it has been compressed by Amazon.com (that's why it is foolish to transfer the file via usb to your device).
I am a big believer in buying the non-DRM file from a publisher or Smashwords rather than buying it from Amazon.com, but if you do that, you need to upload it to your device.
By the way, as someone who produces ebooks, producing .mobi files are a real pain in the neck --although I appreciate that Amazon now is sending you a file size according to what your device can handle.
See also: this article explains why mobi files tend to be so huge and why it doesn't matter for the end user. https://www.52novels.com/from-the-ebook-formatting-files-why-is-my-mobi-file-so-huge/
FURTHER THOUGHTS: I occasionally have issues reading epub files with lots of graphics on Google Play Books or ibooks. Sometimes you're just going to have to live with the file size (especially if the graphics are a vital part of the content), but I trust O'reilly to do as much optimizing as possible. Occasionally you have self-published ebooks by amateurs which make no effort to keep file size down. This raises an interesting question: what file size would be difficult to read on current devices and reading software? Generally, if the epub is over 25 MB, I have found it cumbersome to read on a tablet.