The Sa' (a kind of measure) during the lifetime of the Prophet ﷺ used to be equal to the one Mudd (another kind of measure) and one third of a Mudd which we use today, but the Sa' of today has become large.
سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ يَقُولُ كَانَ الصَّاعُ عَلَى عَهْدِ النَّبِيِّ ﷺ مُدًّا وَثُلُثًا بِمُدِّكُمُ الْيَوْمَ وَقَدْ زِيدَ فِيهِ سَمِعَ الْقَاسِمُ بْنُ مَالِكٍ الْجُعَيْدَ
"I heard As-Sa'ib bin Yazid say: 'During the time of Messenger of Allah, the Sa' was equal to a Mudd and third of the Mudd you use today, and the Sa' of today has become large."' (Sahih) Abu 'Abdur-Rahman (An-Nasa'i) said: And Ziyad bin Ayyub narrated it to me. (Using translation from Nasāʾī 2519)
سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ يَقُولُ «كَانَ الصَّاعُ عَلَى عَهْدِ رَسُولِ اللهِ ﷺ مُدًّا وَثُلُثًا بِمُدِّكُمُ الْيَوْمَ وَقَدْ زِيدَ فِيهِ» قَالَ أَبُو عَبْدِ الرَّحْمَنِ وَحَدَّثَنِيهِ زِيَادُ بْنُ أَيُّوبَ عَنِ الْقَاسِمِ