THC, the active ingredient in marijuana, is “concentrated in the milk (but still present in tiny amounts) ”and chronic exposure could result in the drug accumulating in the baby’s tissues とはどういう意味ですか?

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i am a little confused: why does THC "concentrate in the milk but still present in tiny amounts"? concentrate means high percentages of THC, right? if the milk contain a lot of THC, then how it is in tiny amount?

@alexlen the whole context:If a mother is using marijuana occasionally, and is capable of taking good care of the baby, there is no reason to advise formula. There is no evidence that the small amounts present in the milk will harm the baby.
Chronic heavy marijuana use is a different story. THC, the active ingredient in marijuana, is concentrated in the milk (but still present in tiny amounts) and chronic exposure could result in the drug accumulating in the baby’s tissues. The long- term effects of this accumulation are unknown. The mother should breastfeed, and if she lights up, she should make sure it is only occasionally.

@alexlen so concentrated just means
that the THC could enter and accumulate in the breastmilk(and the author didn't mention whether it's in high or low amounts)?