Sorry for english, I'm from Russia and find your question via google search.
Have same result after try to install buildroot via ipkg.
By mistake press link on your post and see file size is about 42Mb. But at cache of my ipkg is only 10! So, just wget -c it to router and install from local.
wget
buildroot_4.1.1-13_m 100% |*******************************| 42247k 0:00:00 ETA
[......]$ ipkg install buildroot_4.1.1-13_mipsel.ipk
Installing buildroot (4.1.1-13) to /opt/...
Configuring buildroot
Updating /opt/etc/ld.so.cache
Successfully terminated.
All ok.