The gemara discusses whether teruma and ma'asar of dried fruits should be measured based on their present size or on their larger size when they were fresh. The gemara concludes that by Terumah gedola it doesn't really matter because there is no minimum or maximum shiur according to the Torah. However, for Terumas ma'asar which the Torah calls מעשר מן המעשר, the implication is that it must be exactly 10%. Yet, even by terumas ma'aser R. Elazar Ben Gomel says that it is compared to Teruma gedola which can be taken by estimation. Tosafos explains that according to R. Elazar ben gomel the "ma'aser" which implies 10% is a minimum but one is allowed to intentionally increase the amount. Tosafos even raises the possibility that the Rabbonon would agree to R. Elazar Ben Gomel that one may take ma'aser by over estimating, they only argue whether it would be a mitzvah to do so. According to this approach, tosafos explains that the Rabbonon would permit an intentional generous estimation, but wouldn't allow an intentional increase, whereas R. Elazar Ben gomel would hold that one can always intentionally increase even if they realize it is more than 10%.
The Mishna in Avos (1:16) says - ואל תרבה לעשר אומדות. Tosafos understands that this mishna cannot confrom with R. elazar ben gomel because he would allow an intentional increase by ma'aser just as he allows for it by terumas ma'aser, for the same reason that ma'aser is also compared to teruma. However, the Tosafos Yom Tov in avos quotes the maharal that the mishna actually goes with R. elazar ben gomel because the mishna says אל תרבה, meaning that even though one is allowed to take ma'aser by estimation, he should do so as regular practice. This seems to conform with Tosafos that R. elazar ben gomel would allow over estimating even by ma'aser, but would seem to disagree with Tosafos who understands that according to R. elazar ben gomel it is a mitzvah to increase.