Here is a fun tool to play with to estimate the expansion and contraction of wood -
http://www.woodworkerssource.com/movement.php
For example: Let's say that you have an Indian RW small jumbo that was built at 50% RH (Relative humidity). If you moved this guitar to a climate with 90% RH the 15" wide back would swell to 15-3/8"+. Over the winter your home dried out to about 30% RH and now the back shrank to 14-7/8" wide or a total movement of 1/2". Wood is flexible BUT it reaches a point where something has to give. Either a glue joint will fail or the wood cracks because it doesn't have enough elasticity to compensate for the seasonal changes. It quickly becomes apparent why it is important to maintain accurate humidity controls to minimize the risk of movement and eminent failure.
Wood moves the greatest amount in the tangential direction or across the grain or across the back. If your guitar is standing up in the corner then we are talking about possible movement in the left to right direction of the back and top of the guitar. It moves the least amount in the radial [up and down] direction or in the direction of the grain. If more of the back is flat sawn this exposes more flat sawn grain fibers to the air which is more susceptible to tangential movement and faster moisture exchanges with the atmosphere.