Adding 1 byte to struct adds 4 bytes to program

Using Arduino Programming Questions
1

I had the following "struct". When I compile it this way, it leaves 32040 bytes for local variables.

#define MAX_CALLERS 300    // Much more than this and the ESP throws up!  You'll see it on the serial monitor.
typedef struct  // This is where I keep track of which IP addresses have accessed this server.
{
  unsigned long firstSeen; // Epoch
  unsigned long lastSeen;  // Epoch
  IPAddress     iaddr;     // Little Endian
  long          count;     // Times seen
//  byte          ct412s;    // Count of times this guy has 412'd after being told not to. 
} IPCallers;
IPCallers MyCallers[MAX_CALLERS];  // Table for keeping unique IP addresses and access counts.

If I uncomment the "byte ct412s;" the remaining memory is 30840. That's a difference of 1200 bytes. I use "byte" and, since I am defining only 300 of them, it should only use 300 bytes.

Why are the extra 900 bytes going? The first time I did this, I had the "byte" line higher and I thought it might be an alignment thing so I moved it to the end. Is it still an alignment thing?

2

Looks like it. I'll assume an IPAddress is four bytes. So without your byte, that struct is 16 bytes. Adding a single byte jacks it up to 20, so my first thought is that the compiler is aligning on four byte boundaries. See if there's compiler switch you can apply to get byte alignment. Alternatively, keep a separate array for ct412s.