is there a reason they cant make a chip that can use all frequencies?
cost and size?
Because there is more than the chip, you need power amplifiers and antennas specific for each freq.
And one reason the "large phones" appeared to begin with was that the first generation of LTE chips (needed in the US for Verizon) required a LOT of amps of electricity.
I agree those handsets are not my favourite size either, I tend to find blokes holding them in their hands, and women put them in handbags. They don't fit in pockets. One reason to avoid them - but each to their own

James BT Infinity 2 19/09/2012 - Sold 42/6 - Getting 46/10 - Sync 54 / 11 Mbps @ 470m approx
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest