QString and QByteArray memory usage - c++

Question

The QByteArray documentation states that it is to be used when memory conservation is critical. I've been working on a class (FieldName) that keeps an internal store of commonly used QString's so that when two FieldName objects are compared for equality, a quick integer comparison is done.
My thinking was that I could save some memory and gain speed by using QByteArray instead of QString since I only need standard ASCII characters. But when I change out the QString in the following code for QByteArray, my tests show that speed is increased, but memory usage more than doubles. This runs counter to the documentation.
#ifndef H_FIELDNAME
#define H_FIELDNAME
#include <QString>
class FieldName
{
public:
FieldName() : mKey(0) { init(); }
FieldName(const QString& s);
const QString& constString() const;
bool operator==(const FieldName& other) const { return mKey == other.mKey; }
bool operator!=(const FieldName& other) const { return mKey != other.mKey; }
private:
int mKey;
void init();
int findKey(const QString& s);
};
#endif
CPP file:
#include "fieldname.h"
// #include <QMutex> // demo version not thread-safe
#include <QVector>
static QVector<QString*> FieldName__List;
static bool FieldName__FirstCall(true);
FieldName::FieldName(const QString& s)
{
init();
mKey = findKey(s);
}
const QString& FieldName::constString() const
{
return(*FieldName__List.at(mKey));
}
void FieldName::init()
{
if(FieldName__FirstCall)
{
if(FieldName__List.isEmpty())
{
FieldName__List.append(new QString(""));
FieldName__FirstCall = false;
}
}
}
int FieldName::findKey(const QString& s)
{
if(s.isEmpty())
return 0;
int sz(FieldName__List.size());
for(int idx=1; idx<sz; ++idx)
{
if(s == *FieldName_List.at(idx))
return idx;
}
FieldName__List.append(new QString(s));
return(FieldName__List.size() - 1);
}
Can anyone let me know what I may be doing that is causing this to use more memory when I substitute QByteArray for QString in the above class?
Testcode:
QStringList qsl; // filled with 84000 unique English words
FieldName fn;
foreach(QString s, qsl)
{
fn = FieldName(s);
// do stuff here
}