Hah, you've raised a very controversial point, it may true only for the part of companies, but not all of them.
I've made some research on this issue and from the hosting plans of the nrepurable companies I know and mentioned here, I will try to draw a conclusion:
webhostingbuzz.com:
shared plan: 300GB/2000GB -$4.95
reseller plan: 50GB/500GB -$10
ahosting.biz:
shared: 50MB/1000MB -$22
1reseller :1GB/20GB -$6
a2hosting.com:
shared :200MB/2GB - $2.95
reseller 10000MB/100GB -$24.95
onthespothosting.net:
shared: 4GB/200GB- $8.95
reseller 15GB/500GB -$24.95
I think that there's no consistency in the shared/reseller plans ratio in terms of disk space and bandwidth offered and to the greater extent it depends on the host's business plan. In the majority of cases, a reseller plan gives you more resources, which is quite natural as you'll be giving the plans away to people, but reseller plans tend to cost more, as more support is implied in this case.