Okay so is this just to make Dutch people sound cute or does this mean "little cup" so like "Ik drink een kop thee" would be drinking a regular sized average cup of tea, but then "Ik drink een kopjes thee" would be like drinking from a tiny quaint hecka kawaii teacup of cuteness and tininess?
'Kopje' isn't necessarily a super small cup. Both 'een kop' and 'een kopje' refer to a regular sized cup. You wouldn't use 'kopje' when it's a really big cup though. If you wanted to emphasize that it's really a small cup, you could say 'een klein kopje koffie'.
Ps: it's 'ik drink een kopje thee', because thee is not a plural noun.