First off, obviously smaller sizes will always look more detailed, because you're cramming the same amount of information into a smaller area.

With regards to your specific problem, there are a number of factors that could be reducing the clarity of your photos.

1. Mega-zooms like the 18-200 are notorious for being soft. This can be exacerbated by having the lens wide open; try stop down to f8.
2. f/5.6 also gives a shallow depth of field, which is especially noticeable with macro photography. You might simply be a bit out of focus.
3. 1/160s may not be fast enough to eliminate motion blur if there is a breeze or you have unsteady hands. Using a tripod may help.
4. ISO 800 will introduce noise into your images. If you have an entry level DSLR, this can be quite significant. You should generally try to use the lowest ISO possible.
5. All of the settings I mentioned require more light for more detail, so try to shoot during a brighter part of the day or experiment with flash.