For simple economic reasons small parcels, for instance sending just one item, are more expensive. The agents' billing model makes you feel this effect even more drastically.
But what is the maximum you should ship? When does going bigger not make sense anymore? From a financial and safety standpoint the answer is 4kg-6kg is the max you should ship.
After 4kg you are only saving a little bit, but trading off lots of safety for it. This isn't worth it. If you aren't scared of the risks you can go up to 6kg to save a little more (~$4 total). But higher than 6kg doesn't bring any benefits, even if you ship to the safest country in the world.
This is true for almost all shipping lines, because the segmentation and relationship between overhead and unit price is similar. The chart and the numbers used are for Fedex to the United States with Wegobuy in USD.