It is possible that you do not have enough memory allocated to MySQL to complete the dump.
Try editing the MySQL my.cnf configuration file for MAMP. You can do this from the file menu in the application.
These are the values that I use:

[mysqld]
#port = 9999
socket = /Applications/MAMP/tmp/mysql/mysql.sock
skip-locking
key_buffer = 24M #16M
max_allowed_packet = 48M #1M
table_cache = 256 #64
sort_buffer_size = 2048K #512K
net_buffer_length = 32K #8K
read_buffer_size = 1024K #256K
read_rnd_buffer_size = 2880K #512K
myisam_sort_buffer_size = 72M #8M

and later:

[mysqldump]
quick
max_allowed_packet = 128M #16M

If that doesn’t work, increase your PHP memory limit. It is possible that the dump is timing out. These are the values I use in php.ini for php 5:

;;;;;;;;;;;;;;;;;;;
; Resource Limits ;
;;;;;;;;;;;;;;;;;;;
max_execution_time = 600 ;30 ; Maximum execution time of each script, in seconds
max_input_time = 300 ;60 ; Maximum amount of time each script may spend parsing request data
memory_limit = 348M ;8M ; Maximum amount of memory a script may consume (8MB)


don’t forget to restart MAMP after you’ve edited the document.

I tested that back on my MAMP setup, dropping and recreating a new database for each file type, and sure enough only the raw sql file, and the zipped file would result in the original database there too. In both instances the other compression types returned databases with only a small number of the original tables (the first 16 or so).

Really nice tip about the local phpmyadmin url!