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.

static const unsigned int DEFAULT_CHECKLEVEL = 3;
static const uint64_t MIN_DISK_SPACE_FOR_BLOCK_FILES = 550 * 1024 * 1024;
void RegisterNodeSignals(CNodeSignals& nodeSignals);
void UnregisterNodeSignals(CNodeSignals& nodeSignals);
/** 
 * Process an incoming block. This only returns after the best known valid
 * block is made active. Note that it does not, however, guarantee that the
 * specific block passed to it has been checked for validity!
 * 
 * @param[out]  state   This may be set to an Error state if any error occurred processing it, including during validation/connection/etc of otherwise unrelated blocks during reorganisation; or it may be set to an Invalid state if pblock is itself invalid (but this is not guaranteed even when the block is checked). If you want to *possibly* get feedback on whether pblock is valid, you must also install a CValidationInterface (see validationinterface.h) - this will have its BlockChecked method called whenever *any* block completes validation.
 * @param[in]   pfrom   The node which we are receiving the block from; it is added to mapBlockSource and may be penalised if the block is invalid.
 * @param[in]   pblock  The block we want to process.
 * @param[in]   fForceProcessing Process this block even if unrequested; used for non-network block sources and whitelisted peers.
 * @param[out]  dbp     The already known disk position of pblock, or NULL if not yet stored.
 * @return True if state.IsValid()
 */
bool ProcessNewBlock(CValidationState& state, const CChainParams& chainparams, CNode* pfrom, const CBlock* pblock, bool fForceProcessing, const CDiskBlockPos* dbp);
bool CheckDiskSpace(uint64_t nAdditionalBytes = 0);
FILE* OpenBlockFile(const CDiskBlockPos &pos, bool fReadOnly = false);
FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly = false);
boost::filesystem::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix);
bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp = NULL);
bool InitBlockIndex(const CChainParams& chainparams);
bool LoadBlockIndex();
void UnloadBlockIndex();
bool ProcessMessages(CNode* pfrom);
/**
 * Send queued protocol messages to be sent to a give node.
 *
 * @param[in]   pto             The node which we are sending messages to.