In the case of pants like jodhpurs, there isn't any fancy trickery inside the pants - they are simply cut that way.
Since the reference picture is of a SD style drawing, I would have to ask first if you're making the whole costume over-exaggerated in that style. If not? Then it won't be an issue. I've seen other ninja characters with the same basic style of pants, and they're not hardly poofy at all. If they are, it's simply due to the volume of the pant leg being tucked into the top of boots or leg armor. They aren't thin, narrow, or tapered pant legs, they actually have a little volume in them, but it's natural. It's also natural for the pants to puff a little at the knee if you have something there, such as an armor strap or the top of a boot, constricting the fabric at that point. Essentially, gathering it.
But if you ARE planning to really over-exaggerate the clothing to make it closer to the SD style, I would think all you need to do is cut the pant legs wider than usual and gather them into whatever knee or lower-leg boot/armor you've got going on there. They're not jodhpurs or slops (pumpkin pants), so elaborate inner stuffings and interfacing isn't necessary. Now, if you were making pumpkin pants, that would be an entirely different story.
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