Why are some transactions larger (in bytes) than others? There's this transaction: http://blockchain.info/tx/2588df59035129de0f0cb20d2cb5d77d18830043aa68ad2a1a2df7d727bc7a62 that is 802 bytes but for only 1.4BTC

Another transaction: http://blockchain.info/tx-index/14803514/781333d877bae2b4d8fc7ff856b38989013f0a11668395b8f90b423dd2ffc230 is for 50.4BTC by only 118 bytes.

So what exactly goes into a transaction that would increase / decrease the size of the transaction?
I give a formula that you can use to calculate the size of a transaction given the number of inputs and outputs here.

The most significant factor contributing to the size of a transaction is the number of inputs to transaction has.  Each input contributes 179 to 181 bytes to the size, and each output contributes 34 bytes to the size.

Your large transaction has 4 inputs - that's why it's bigger than the other transaction, which has none.

The actual number of bitcoins spent by a transaction is completely irrelevant, except that there may be a tendency for transactions which spend more coins to require more inputs to make up the amount.