Well, my Core i3-530 build uses around 80W full load (AC) so I'm going to take a guess that it doesn't really use 73W unless under extreme conditions - the TDP is really just a guideline for coolers. Besides, the 73W is supposed to cover both the CPU and integrated GPU (H55/H57/Q57 chipsets have pretty low TDP). With your 780G build, you also need to take into account the 780G chipset and HD 5450. Motherboard implementation can also make a pretty big difference. MSI is usually pretty good when it comes to power consumption.
Hmm, also, are you sure your build uses 100W at load? How many hard drives do you have installed? Also, what power supply? Assuming that's 100W from the wall, if the PSU is only 70% efficient, then actual power consumption is just 70W.
I'm using a Winmate with a 160w brick. System is a Foxconn 780G board with 2GB RAM, Athlon 4850e stock, 1X Blu Ray drive, 1X WD15EADS 1.5TB 5400RPM drive, ATi HD 5450, Big Shruiken, Bluetooth keyboard and USB wifi dongle. Idle it is at about 40w, under real-world load it is at about 70-90w. Under synthetic PSU stress test programs I got it up to 100w. Measuring from the wall with a Kill-a-watt.
Anyway I guess that this thread is less about my current system and more about figuring out whether I would be able to use a PicoPSU to run a Mini ITX system for an HTPC based on a Core i3 530.