there are different types of tapeworms that infect different hosts. cattle tapeworms can grow to 12 feet and much more. It doesn't take much googling to find claims of 100 feet.
google "tapeworm symptom loss of weight" and you will get the recommendation "time to see a doctor"
That's part of it, but intestinal parasites also interfere with normal digestive processes. Their presence and secretions (waste products and the compounds they use to prevent being digested) can cause a lot of problems. It's common for people with intestinal parasites to have reduced appetite, intestinal inflammation, and digestion issues.
The weight loss isn't just a result of the parasite competing for nutrients, though that doesn't help.
I thought of a refinement to my previous: tapeworms attach themselves to the host intestinal wall only to secure themselves; however, they feed directly from the food in the digestive tract; they do not "suck nutrients" from the host's bloodstream.
I'm no expert on invertebrate metabolism, but it does not need to be more expensive than host metabolism, it's simply a redivision of the pizza-pie with more slices going to the growing worm, and fewer slices for the host.
consider the possibility of there being n worms in the host (person), where n is greater than 1, maybe even significantly greater than 1.
so if one worm takes m slices of the pie, and there are n worms in the person, there will be m x n less slices available for the person, which, depending on the values of m and n, could be quite high, and therefore much more detrimental to the person
google "tapeworm symptom loss of weight" and you will get the recommendation "time to see a doctor"