It's not 8 "because it looks like boobs". It's because Japan has a way to substitute words/characters by using numbers. Refer to the Numeric substitution
Here for further details, but short explanation for the number 1108 is:
1 = 一 (いち or ichi) is shortened to i (so two 1s is ii)
0 = is read as if it were an O
8 = 八 (はち or hachi) = shortened to "ha". "ha" is the base character for "pa"
So combined that's "ii opa" then they just adjust it with an additional i at the end to make it ii oppai.
Other sample dates are
1104 = ii oshi (changed to ii oshiri or Nice Butt)
1107 = ii ona (changed to ii onaka or Nice Belly)
Sample words used in ecchi settings are:
19 = iku (cum)
45 = shiko (fap)
By the way, a stream of the number 8 is also used as shorthand for clapping because Japanese SFX for Clap is Pachi (which is shortened to "pa" similar to the case explained earlier) so
8888 = 👏👏👏👏