How do you explain that everyone I know(I'm from Bosnia, rep. of Srpska) is looking so much Meditteranian? Even tho we're Slavic. I'm 6'4'', and everyone in my family is above 6'0'', from my experience other people, for example Russians, Polish, they're much shorter, and lighter-haired, fair skin... We're pretty dark, curly hair mostly(it is true that I was born blond with fairly light coloured eyes, but my hair got dark brown over the years and my eyes are almost black). When I meet Greeks, I feel as if they're my family(even tho they tend to be quite shorter and more fat). Tendency in my family in general is that we're very skinny and men get and stay muscular after 30's. Maybe the question is ridiculous, but these things always got me thinking.
Your question is probably better suited for some genetics subreddit, although there are already some good discussions on the origins of Slavs:
What is a Slav, specifically the comment from u/Freevoulous
What are the origins of the Slavs? How did they populate much of Eastern Europe? u/rusoved
To answer your particular question, the reason the Greeks looks like family to you is because they are closely related genetically. Despite the reputation of the Balkan peninsula as "The Crossroads of Empires", "The Invasion Gateway of Europe", etc. and all the local nationalism, the dominant genetic components in the Balkan peninsula are predating the Bronze age era, based on the early farmers migrations from the Middle East. The Slavic genetic component varies between 10% and 20% in the countries in the region (including Northern Greece).
"Standing at the Gateway to Europe - The Genetic Structure of Western Balkan Populations Based on Autosomal and Haploid Markers" - Kovacevic; et al.
"Y-Chromosome Diversity in Modern Bulgarians: New Clues about Their Ancestry" - Karachanak; et al.