Why are some transactions larger (in bytes) than others?
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.