this question is on genetic drift in biology alevel aqa.
why does genetic drift affect smaller populations more than large ones?
It might make more sense if you think about an example such as a captive breeding program. In my breeding program, I have 20 adults who can reproduce- at the moment let's pretend every animal is Bb for coat colour- so there's an even mix of each allele.
You'd normally expect 25% of offspring to be BB, 50% to be Bb and 25% to be bb- but in a small population things might not perfectly follow this pattern, so say in the next generation, 20% are BB 40% are Bb and 40% are bb.
This is much more likely to happen in a small population where, say, only 10 offspring are born each year than in the large wild population where 100 offspring are born each year- in a large population random chance is more likely to balance things out.
So in the next generation maybe only 10% are BB, 50% are Bb and 40% are bb- already we're losing lots of B alleles.
If you had lots of alleles for a gene, and one was already quite rare, it could have been lost by this stage- if you've got 20 individuals, and the allele is present in 5% of individuals, that's only one individual with the allele. In a population of 1000 that's 50 individuals with the allele, so it has a much greater chance of being passed on.