Tosefta על כלים 1:3

There is a stringency with corpse-impurity that there is not with a ba'al niddah (one who has intercourse with a menstruant), and [there is a stringency] with a ba'al niddah that there is not with corpse-impurity. That [one contaminated by] corpse-impurity requires sprinkling on the third day and the seventh day, which is not the case with a ba'al niddah. The stringency with a ba'al niddah is that a ba'al niddah defiles the bottom [bedding] just as the top bedding, which is not the case with corpse-impurity. Rabbi Yosei says, the emission of a zav, his spit, his semen, and his urine, [and the blood of a niddah (per GR"A)], and a revi'it of blood of a corpse [also defile both the top and bottom bedding]. Why did they say a "revi'it"? Because anyone who crosses the threshold of creation has [at least] a revi'it of blood. And regarding the blood of a niddah and [contamination with] riding-uncleanliness (Lev. 15:9, see Jastrow ("מרכב")), there is a stringency with [a niddah] that there is not with riding-uncleanliness, and with riding-uncleanliness that there is not with [a niddah]. That [the niddah] is equivalent to one who renders a person impure or clothing impure by contact or by carrying, which is not the case with riding-uncleanliness (Kel. 1:3). The stringency with riding-uncleanliness is that one who possesses riding-uncleanliness can impart impurity [even] under a heavy stone, which is not the case with [a niddah].