Magento2 – Why is Size of a Collection Greater Than the Number of Products

Take a look at the definition of getSize (it's in \Magento\Framework\Data\Collection\AbstractDb). You'll see the following:

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

The method getSelectCountSql does exist in the original class, but if you follow it down:

public function getSelectCountSql()
{
    return $this->_getSelectCountSql();
}

To:

protected function _getSelectCountSql($select = null, $resetLeftJoins = true)
{
    $this->_renderFilters();
    $countSelect = is_null($select) ? $this->_getClearSelect() : $this->_buildClearSelect($select);
    $countSelect->columns('COUNT(DISTINCT e.entity_id)');
    if ($resetLeftJoins) {
        $countSelect->resetJoinLeft();
    }
    return $countSelect;
}

To:

protected function _buildClearSelect($select = null)
{
    if (null === $select) {
        $select = clone $this->getSelect();
    }
    $select->reset(\Magento\Framework\DB\Select::ORDER);
    $select->reset(\Magento\Framework\DB\Select::LIMIT_COUNT);
    $select->reset(\Magento\Framework\DB\Select::LIMIT_OFFSET);
    $select->reset(\Magento\Framework\DB\Select::COLUMNS);

    return $select;
}

You'll see that getSize will always reset the LIMIT and perform a MySQL query to give you the size of the full collection (assuming I didn't miss somewhere that sets _totalRecords outside of that method). Given that you've already loaded the collection, calling count($collection) should result in the right number.

The issue was due to conflict between Search Result and Amasty Sorting module.

Amasty Sorting module extends available search filters on products list and search result pages.

Amasty has mentioned on their documentation that it doesn't support solr but not sure why.