In some Spanish speaking cultures, maybe all, the size of a computer file is referred to as its weight.
Instead of wondering how large that particular music video you'd like to download is, you wonder how heavy it is.
Rarely is this usage seen in English speaking cultures. Though at times, it could come in handy. For example, with regards to an image file, "size" may refer to the number of bytes in the file or to the image's dimensions when displayed. Each of these properties is rather independent of the other (okay, not really). In Spanish, these properties can be referred to as "peso" (weight) and "dimensión" (dimension) respectively.
English speakers have made up for the ambiguity by creating the term file size to specifically refer to the number of bytes in a file. In my humble opinion, Spanish speakers win the prize for elegance by not creating a new term when an old one does a great job.
jmpz has just informed me that no Spanish speaking country he's familiar with (Spain, Puerto Rico, Venezuela, Chile and Mexico) has this colloquialism. My sources are Bolivian (specifically Camba, persons from the region of Santa Cruz de la Sierra), where it is definitely common. It is often the case with colloquialisms in Spanish cultures that a common word in one region will have different/no meaning in even a bordering country.
motiz88 says re weight: Nice. It happens here in Israel too, although not as commonly. I'm pretty sure it's partly due to people confusing kilograms and kilobytes... You might want to add something about that... That's a good explanation for it. Kilo is a shortened form of many measurement terms. I believe this is true for most languages in countries that use the metric system. If this is the case, it explains the fact that some/many Spanish speaking countries do not use "peso" this way, as using it was a mistake in the first place.