If we want to cap the time of downloading overhead the latest block to say 1%, we need to be able to download the MAX_BLOCKSIZE within 6 seconds on average so that we can spend 99% time hashing.
At 1MB, you would need a ~1.7Mbps connection to keep downloading time to 6s.
At 10MB, 17Mbps
At 100MB, 170Mbps
and you start to see why even 100MB block size would render 90% of the world population unable to participate in mining.
Even at 10MB, it requires investing in a relatively high speed connection.
Also of importance is the fact that local bandwidth and international bandwidths can wary by large amounts. A 1Gbps connection in Singapore(
http://www.starhub.com/broadband/plan/maxinfinitysupreme.html) only gives you 100Mbps international bandwidth meaning you only have 100Mbps available for receiving mining blocks.
Since a couple people have thanked the author for posting this, I thought I should mention that only transaction hashes need to be sent in bursts. So a block of 1000 transactions (roughly 1MB) only requires 30KB of data to be sent in a burst, requiring a ~43Kbps connection to keep downloading time to 6s. 100MB blocks require ~4.3Mbps. The continuous downloading of transaction data is below these limits.