Disclosure: Google employee
[0]https://abc.xyz/investor/static/pdf/2020Q1_alphabet_earnings... [1]https://www.apple.com/newsroom/pdfs/FY20_Q2_Consolidated_Fin...
Activist investor shows up with $100M fund that they can throw behind Yelp giving them ~10% of the company, they'll have a major say and in some other public companies they might hold the largest share stake depending on the shareholder splits.
The same investor could show up with $100M on a $1T company, have 0.01% and nobody even knows who they are and they can't really push the company to do anything.
There are not that many funds that can take major voicing stakes in GOOG or APPL because of the capital it would require.