How can I minimize the content of my RSS feed to fit more items in Feedburner’s 512k limit?

Dunno if you have images in your feed, but those could be sucking up a bit of space: Reducing image size in RSS only

Here are the basics for setting WP to use a custom feed template: http://www.456bereastreet.com/archive/201103/controlling_and_customising_rss_feeds_in_wordpress/

Here are some tips in authoring the template itself: http://digwp.com/2009/09/easy-custom-feeds-in-wordpress/

As for your looping, try something like this for your “full” 75 most recent posts (inside your RSS structure of course):

<?php
query_posts('showposts=75');
$ids = array();
while (have_posts()) : the_post();
$ids[] = get_the_ID();
the_title();
the_content();
endwhile;
?>

Once that’s done, get all the rest, with just truncated info:

<?php
query_posts(array('post__not_in' => $ids));
while (have_posts()) : the_post();
the_title();
the_excerpt();
endwhile;
?>