What size are those shots? Just about every country has its own shot size, and "a beer" could be .33l or .5l.

That said, I experience similar effects even if I make sure that the alcohol amounts are equivalent. Something about beer makes it go right to my head, unlike spirits. IIRC both sugar content and carbonation increase the speed at which alcohol gets absorbed, but I don't get this effect with longdrinks, either. Maybe it's just placebo ...