I was just looking for the same information and came upon three potential explanations from different dermatologists (some mentioning more than one possibility), and then I finally found Dr. Cynthia Bailey, who mentioned all three, so I'm going to hope that her's is the most complete and thus accurate. An excerpt from her website states...
"The solution to treating bumps under the eyes needs to start with an accurate diagnosis, which means a trip to a dermatologist! The reason is that there are several other alternative causes of “bumps” under the eyes, and they are more common than clogged pores in this area.
Common causes of bumps on the lower eyelid skin:
#1 - Prominent sebaceous glands. There are many sebaceous glands in this area and they become visible when the eyelid skin thins with age or becomes dark due to circles under the eye.
#2 - Syringomas. These are actually benign tumors of the sweat glands. To remove or reduce these in size requires surgical or laser treatment.
#3 - Milia. These are tiny white cysts that can form anywhere, including the eyelids. They can easily be lanced by someone trained in blood-born precautions (e.g. medical personnel)."
As for the difference between a serum and cream, it’s actually not a hard and fast difference. In fact, the term is often randomly selected by the product company for market niche purposes! To really evaluate the pore-clogging capacity of a product, you have to read the label. Look for oils and, if you want something less apt to block pores, skip the oils. An interesting option would be to try a pure hyaluronic acid serum [product name omitted to avoid outside advertising]. It is very light and the hyaluronic acid hydrates skin to plump it, possibly diminishing the appearance of sebaceous glands. It will not lighten dark circles though. [A different product] has ingredients that can lighten dark under-eye circles. It contains retinol, hyaluronic acid, and no oils. It is an opaque product that looks creamy, hence the term “cream.”
Finally, it’s important to know that retinol can irritate skin. That’s why you need to figure out the right strength for your eyelid skin. Avoid using a retinol product that’s too strong for your delicate eye area because it can lead to irritation, which unfortunately can lead to milia formation. I’ve found that the retinol level in [the product she uses] is right for most people’s skin, which is another reason that it’s the eye cream that I use in my practice, and that I use for my own sensitive skin."
Long story short, my own non-professional, but suffering the same condition, advice is to remember that your skin is the largest organ of your body. Seeing a dermatologist is not a luxury like a spa appointment. It is a doctor visit. And many (if not most) healthcare plans with cover at least an initial visitation (especially if you haven't had a cancer screening) to access the nature of "unidentified bumps." So do the right thing for your very real health condition and save yourself from missing something dangerous and the potential of making it worse.
I hope this information helps you as much as it helped me.