The first phones that integrated a camera were capable of shooting photos in VGA resolution if you were lucky. The listed specs of those phones included an internal memory is just 2 MB, which could only store about 10 pictures in that resolution. Some of them didn't have Bluetooth connectivity, so if you wanted to store your photos somewhere else, you'd have to buy a proprietary cable because most companies didn't include them back then. Then, phones with 1-megapixel cameras started appearing and a memory card slot was getting more popular among phones. Finally, Micro SD cards were accepted as a standard and 128 MB was their largest capacity. In 2006, phones were capable of taking 5-megapixel pictures and screens were large enough to make you want to install a few apps or games, thankfully cards up to 8 GB were supported.
Let's jump to the present, where we have cameras in our phones that are more capable than most point-and-shoot cameras and with sensors as big as 20 megapixels (with a few exceptions that have even bigger sensors). In the Android world, there's a new acquired support for images taken in RAW format, so those files are even larger than an average photo. Let's not forget about the apps, some graphically intense games could take up to 2 GB of storage and some apps like Pocket could end up storing files that take some more space.
While there are some phones that do not have Micro SD card support, the ones that do, take cards up to 128 GB. SanDisk has announced one with a capacity of 200 GB but a company called Microdia seems to think that capacity is no longer enough and will release a Micro SD card with the unprecedented capacity of 512 GB next July costing something around $1,000. It's in the newer format of SDXC that could have cards with a capacity up to 2 TB and it supports transfer speeds up to 300Mbps. The card is not meant to be for smartphones but rather professional photographers that could take advantage of this kind of speeds and capacity, but I think it's pretty clear where the future is heading and sooner or later we will need that much space in our mobile devices.