"Mis abuelos son un poco bajos."
Translation:My grandparents are a little short.
If someone told me they were a little short I would assume they meant they just don't have enough money for something. Figure the literal meaning is what we're going for here, unless the figurative meaning translates as well?
Short people can be fat, so "small" may not always be a good synonym for "short," although sometimes it can be if the people are both short and thin.
I think it's because 'poco' is quantifying 'bajo' rather than saying that the 'abuelos' themselves are pocos.