The resolution determines the render time and dpi settings for print size doesn't add up to time spent.
When using Maximum Time, you define the render time. The total time isn't controlled by the image size or render settings, but simply how much time you allow it to take. The results may vary depending on how long you let it take, usually more time means better quality (to a point). But using Maximum Time is very similar to letting the realtime window sit and render, you're simply the one who stops it after a set amount of time.
Maybe set to a jpeg file will match the file size from the original renders and probably cut rendering time also. .
This isn't true at all; the render time is the same regardless of the type of file you save. Choosing a PNG, JPG, TIFF, etc has NO effect on your render time, it's simply defining the type of image that is saved.
Again, to render this image in a shorter amount of time, simply choose less time under your Render Options.
If you're seeing grainy results, try increasing the amount of time, or the number of samples on the materials themselves.
