P =

Here's an interesting definition of "portability", a concept with a lot of similarities with "mobile". It refers to products, only products.
P = "T" (usage time of a single charge) by "U" (ubiquity of usage: 1 only indoor 5 indoor and outdoor) divided by "w" (the weight of the product in grams).
According to this index, the most portable product, among a 17 range products considered, has been a Bluetooth earphone, with a 4,6 index. At the other extreme you can find the 17.inch laptop, with a 7,500 times lower index (0.0006045).
More details about this model and more things about portability in "Portable Consumer Electronics", by S. Canumalla and P. Viswanadham.