The rabbis discuss this very question on our page.
It is clear that some measurements the rabbis discuss are empirically larger than others. The argument is that in some cases, like the sukkah, a cubit is 5 handbreadths while for others, like kilayin (the distance between crops so that they are not sown together) a cubit is 6 handbreadths.
Raba, however, stated in the name of R. Nahman: All cubits [prescribed for legal measurements are] of the size of six [handbreadths], but the latter are expanded while the former are compact.
That is, for some measurements the hand is held closed and for other it is opened wide.
The question is also raised about the legal authority of the measurements. Do they actually come from Torah, or are they part of the Oral Tradition:
R. Hiyya b. Ashi stated in the name of Rab: [The laws relating to] standards, interpositions and partitions [are a part of] the halachic code [that was entrusted] to Moses at Sinai (i.e. Oral).
Are [not the laws relating to] standards Pentateuchal, since it is written in Scripture: A land of wheat and barley etc. (Deut. 8:8) and R. Hanan stated that all this verse was said [with reference to the laws] of standards?
That is, are these laws derived directly from written text – as in Then shall he bathe all his flesh (Lev. 15:16) – the all indicating a minimum measurement of water and the laws of interpositions (nothing can come between the waters of the mikve and the individual)? Or are they laws the rabbis created and then found legal justifications in Torah text? For example the specific measurements in the rabbinic texts?
Do you then imagine that the standards were actually prescribed [in the Pentateuch]? [The fact is that] they are but traditional laws for which the Rabbis have found allusions in Scripture.