in v0.12 https://github.com/bitcoin/bitcoin/pull/7081 (by @luke-jr) was merged adding `-bytespersigop` (with default of 20).

this is preventing simple bare multisig transactions from being accepted into mempool when the size is < 400 bytes.  
eg; https://gist.github.com/rubensayshi/ac4f617207f7a50559e85d61c05800be

from 1-of-7 and onwards they are accepted because the ratio of bytes:sigops is high enough.  
also having 2 or more inputs will work because it again tips the ratio of bytes:sigops over.

the reason mainly being that `GetLegacySigOpCount` calls `GetSigOpCount` with `fAccurate=false`.

It looks like changing `GetLegacySigOpCount` to use `fAccurate=true` would cause side effects in the few other places (`ConnectBlock`, `CheckBlock`, `CreateNewBlock`) and simply adding `fAccurate` as arg to `GetLegacySigOpCount` would affect the value stored in `CTxMemPoolEntry.sigOpCount` which in turn affects a bunch of other things.

int expired = pool.Expire(GetTime() - age);
    if (expired != 0)
        LogPrint("mempool", "Expired %i transactions from the memory pool\n", expired);
    std::vector<uint256> vNoSpendsRemaining;
    pool.TrimToSize(limit, &vNoSpendsRemaining);
    BOOST_FOREACH(const uint256& removed, vNoSpendsRemaining)
        pcoinsTip->Uncache(removed);
}
std::string FormatStateMessage(const CValidationState &state)
{
    return strprintf("%s%s (code %i)",
        state.GetRejectReason(),
        state.GetDebugMessage().empty() ? "" : ", "+state.GetDebugMessage(),
        state.GetRejectCode());
}
bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const CTransaction& tx, bool fLimitFree,
                              bool* pfMissingInputs, bool fOverrideMempoolLimit, const CAmount& nAbsurdFee,
                              std::vector<uint256>& vHashTxnToUncache)
{
    const uint256 hash = tx.GetHash();
    AssertLockHeld(cs_main);
    if (pfMissingInputs)
        *pfMissingInputs = false;
    if (!CheckTransaction(tx, state))
        return false;