To pick a random 13" MBP, such as this one from mid 2012:
The 8Gb limit seems to be an Apple supported limit, but many people happily report getting 16Gb working fine - see the memory section in the link as well as this page for details.
The "Mid-2009" and "Mid-2010" MacBook Pro models support 1066 MHz PC3-8500 DDR3 SO-DIMMs and officially and unofficially support a maximum of 8 GB of RAM with one odd exception.
Specifically, as discovered by OWC, the 13-Inch "Mid-2010" MacBook Pro models (and only the 13-Inch models) -- the MacBook Pro "Core 2 Duo" 2.4 13" Mid-2010 and "Core 2 Duo" 2.66 13" Mid-2010 identified by MacBookPro7,1 -- can support up to 16 GB of RAM with dual 8 GB memory modules if they are running OS X 10.7.5 or higher, have been updated to use the latest EFI, and are equipped with proper specification memory modules. If running a version of Mac OS X 10.6 "Snow Leopard," these models only can use 8 GB of RAM.
The "Early 2011" and "Late 2011" MacBook Pro models use faster 1333 MHz PC3-10600 DDR3 SO-DIMMs. They officially support 8 GB of RAM, but OWC has determined that they actually support 16 GB of RAM regardless of the operating system installed, although only the "Early 2011" MacBook Pro models are capable of running a version of Mac OS X 10.6 "Snow Leopard."
Finally, the "Mid-2012" non-Retina Display MacBook Pro models use faster still 1600 MHz PC3-12800 DDR3 SO-DIMMs and likewise officially support 8 GB of RAM, but actually can support as much as 16 GB of RAM. These models do not run versions of OS X prior to OS X 10.7 "Lion."