This is such a common mistake, which I have personally seen A LOT!
This is mainly with the guys, but I have seen the odd girl do this. Guys tend to get in the gym and want a big chest and big arms, but that soon develops into an overall big 'upper body'. The problem is, they start to neglect their legs. This can cause a few problems...
Yes, its great to have bulging biceps, chiseled chest, washboard abs and a strong back, but as soon as you are in just shorts (or even worse - swimming trunks!) you will look ridiculous if you have neglected training your legs. There is nothing more embarrassing than your top half looking like a tank, but your legs look like twigs. Not only will this just look ridiculous (and probably quite funny for some) but it can actually cause an injury very easily.
Think about it, if your top half weighs a fair bit because of a lot of muscle, and your walking around on weak legs, that will put a lot of pressure on your joints, and that is not good. You will never see an athlete who has weak legs. Some of the biggest power lifts you can do, involve your legs. Squats and dead lifts are extremely common, and they both require and work your legs. Don't be a fool, make sure you include either separate leg sessions into your workout plan, or include leg exercises into your full body workouts. Not only will you not look silly on the beach, but you will be far less likely to get an injury.