That's because new shares aren't 1MB yet as your fork allows. Then again you advocate making Bitcoin's block size limit infinite...
New shares are already as big as they're going to get in the absence of explicitly malicious behavior or Bitcoin blocksize increases.
Do you mean in the sense that nobody has attacked the p2pool network with large, difficult-to-propagate shares, as an adjunct to doing a selfish mining attack, because somehow that's easier than just renting a bunch of hashrate on Nicehash and 51% attacking p2pool?
Or intentionally creating large shares with the intent of forcing low-bandwidth miners off of p2pool and onto other pools because somehow there's an incentive for someone to want to do that?
Keep in mind that with a 1 MB block size limit, the only way to have shares that require 1 MB of network traffic is to create block templates that use transactions that nobody has heard of before (i.e. transactions you created yourself). This also requires hacking p2pool so that it does not notify peers of transactions when they're first seen (i.e. disabling the have_tx p2p message). Normally, shares require only 2 to 32 bytes per transaction when transmitted regardless of the transaction size, but the 50 kB/1 MB limits apply to the total summed size of the transactions (e.g. 500 bytes).
Even to the extent these attacks are possible, they wouldn't be very effective. If you want to DoS someone, there are much easier ways than mining big shares.