Several years clogged of hair follicles by sebum can do miniaturization of hair in the donor area ?

Dyaf,If I understand you correctly, the Dermatologist found 40% to the follicles in the donor area to be miniaturized. If this is the case then you are developing Diffuse Unpatterned Alopecia (DUPA) which means that your donor area is not stable and will be prone to thinning. I would recommend that you consider maximizing your medical regimen to possibly including: Propecia, Rogaine 5%, Nizoral, Biotin, and low level laser light therapy. All the best,Bernardino A. Arocha, MD

DHT causes miniaturization. Rarely DHT causes miniaturization of the hairs in the back of the scalp - a phenomenon called DUPA (diffuse unpatterned alopecia). DHT does affect lipid metabolism but not in any significantly meaningful way in the scalp

While there are many causes of hair loss, the most common cause is the genetic male pattern hair loss.  You need to be evaluated by a hair restoration specialist to find the cause and proper treatment options.