Difference between getSize() and count() on collection

Most (if not all) the collections extend Varien_Data_Collection_Db. Here are the 2 methods from this class

public function getSize()
{
    if (is_null($this->_totalRecords)) {
        $sql = $this->getSelectCountSql();
        $this->_totalRecords = $this->getConnection()->fetchOne($sql, $this->_bindParams);
    }
    return intval($this->_totalRecords);
} 

public function count() //inherited from Varien_Data_Collection
{
    $this->load();
    return count($this->_items);
}

There is a difference. For getSize() the collection is not loaded. For count() it is. Usually collection models use the same getSize() method as above and only override getSelectCountSql().
In getSelectCountSql() the limit is reset in order to get the total number of records available for the set filters (where statement). See how the getSelectCountSql() works

public function getSelectCountSql()
{
    $this->_renderFilters();
    $countSelect = clone $this->getSelect();
    $countSelect->reset(Zend_Db_Select::ORDER);
    $countSelect->reset(Zend_Db_Select::LIMIT_COUNT);
    $countSelect->reset(Zend_Db_Select::LIMIT_OFFSET);
    $countSelect->reset(Zend_Db_Select::COLUMNS);
    $countSelect->columns('COUNT(*)');
    return $countSelect;
} 

Be careful. This is correct, but the methods are overwritten in Varien_Data_Collection_Db as described by Marius

Just have a look into

// \Varien_Data_Collection::getSize
public function getSize()
{
    $this->load();
    if (is_null($this->_totalRecords)) {
        $this->_totalRecords = count($this->getItems());
    }
    return intval($this->_totalRecords);
}

// \Varien_Data_Collection::count
public function count()
{
    $this->load();
    return count($this->_items);
}

So it should on this low level be the same. Both methods load the collection and count the items.

UPDATE

Oh I see a problem: getSize() caches the _totalRecords, this means it is not recalculated. Check where _totalRecords is set?

This answer shows up in google for "magento getSize wrong" and similar searches so I would like to add a possible scenario that might be useful to someone

When you have a group statement in your query and you do a

SELECT COUNT(DISTINCT e.entity_id) ... GROUP BY ( at_id_art.value )

Mysql will return a count for EACH of the groups, so Varien_Data_Collection_Db::getSize() will return the wrong answer, this is because this function fetches the first row:

public function getSize()
{
    if (is_null($this->_totalRecords)) {
        $sql = $this->getSelectCountSql();
        $this->_totalRecords = $this->getConnection()->fetchOne($sql, $this->_bindParams);
    }
    return intval($this->_totalRecords);
}

When it populates

$this->_totalRecords = $this->getConnection()->fetchOne($sql, $this->_bindParams);

It selects the first row and therefore returns the total of the first group as the total size.

I ended up coming up with this code to count, based on the unique values of the attributes in my query.

$select = clone $collection->getSelect();
$group = $select->getPart(Zend_Db_Select::GROUP);
$select->reset(Zend_Db_Select::GROUP)->reset(Zend_Db_Select::COLUMNS)->columns("COUNT(DISTINCT {$group[0]})");
$totalCount = $collection->getConnection()->fetchOne($select);