I’m super impressed by the smooth vector art shown in the image below. I’m aware of the Illustrator blend tool, but I did not even get close to the desired effect.
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First, add objects for intermediate steps.
If you examine your sample it blends from medium > small > large > small > medium. There are 5 stages to that blend. So you need 5 objects.
Here I've shown the intermediate steps as red circles. And then created the blend. Remember stacking order matters. Blend stack, by default, from the top object to the bottom object. So my circles are stacked left to right, top to bottom.
Then, once you have the basic blend, you can use the Pen Tool to alter the spine - creating curves. You can also adjust the base objects as necessary to alter the overall shape of the blend.
Here's a horribly grainy animation (to keep the file size down) showing alteration of the blend spine....