The button has a default padding inline of 1.75ch and a default padding block of 0.75ch. You can change this value by using the --o_padding-inline and --o_padding-block custom properties.
1.75ch
0.75ch
--o_padding-inline
--o_padding-block
.o-btn { --o_padding-inline: 1ch; --o_padding-block: 0.5ch;}
.o-btn-fat { --o_padding-inline: 5ch; --o_padding-block: 2ch;}