The first thing to understand about saving to jpeg is that one thing is understood: you want to compress your image (and in the process throw away image data). You may not understand this, but by design jpeg conversion software does.
So, when you save a jpeg, the software you use will have certain default conversion/compression settings in place. If you are sure that you want to execute a new compressing of your image for, say the Web, then you need to carefully pick from your software's settings for your own purposes. Most Image software have setings such as quality/compression and give you some idea of what the outcome will be in other terms, such as file size.
I mostly use Lightroom to export pics for the Web, and it makes things pretty simple: it ensures that my images are converted to the sRGB color profile and I can constrain them to pixel dimensions: I prefer a maximum of width = 800 and height = 640. I don't need to worry about file size or other unknown quantities. And for Web viewing it's great. But, if I wanted to email a high-quality image to, say, a person who had a need to have a critically sharp view of a pic, then I would take a different approach (and tweak my image accordingly).
It's not all that complicated, just things to bear in mind!