The font size of an element can be reduced to 80% using the small class, or by wrapping text in the small tag (refer to Figure 8.8):
<small>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam , quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</small>
Figure 8.8: Reducing text size using the small class; note how the text size in the first paragraph is much smaller than the size of the text in the second paragraph
Source: Jakobus Benjamin, Marah Jason (2018), Mastering Bootstrap 4: Master the latest version of Bootstrap 4 to build highly customized responsive web apps, Packt Publishing; 2nd Revised edition.