diff --git a/spiOverJtag/Makefile b/spiOverJtag/Makefile
index d66ac82..9443361 100644
--- a/spiOverJtag/Makefile
+++ b/spiOverJtag/Makefile
@@ -2,7 +2,7 @@ XILINX_PARTS := xc6slx45csg324 xc6slx100fgg484 \
 	xc7a35tcpg236 xc7a35tcsg324 xc7a35tftg256 \
 	xc7a50tcpg236 xc7a75tfgg484 xc7a100tfgg484 xc7a200tsbg484 \
 	xc7s25csga324 xc7s50csga324
-XILINX_BIT_FILES := $(addsuffix .bit,$(addprefix spiOverJtag_, $(XILINX_PARTS)))
+XILINX_BIT_FILES := $(addsuffix .bit.gz,$(addprefix spiOverJtag_, $(XILINX_PARTS)))
 
 ALTERA_PARTS := 10cl025256 ep4ce2217 ep4ce1523 5ce223 5ce423
 ALTERA_BIT_FILES := $(addsuffix .rbf.gz, $(addprefix spiOverJtag_, $(ALTERA_PARTS)))
@@ -11,8 +11,8 @@ BIT_FILES := $(ALTERA_BIT_FILES) $(XILINX_BIT_FILES)
 
 all: $(BIT_FILES)
 
-$(XILINX_BIT_FILES) : spiOverJtag_%.bit : tmp_%/spiOverJtag.bit
-	cp $< $@
+$(XILINX_BIT_FILES) : spiOverJtag_%.bit.gz : tmp_%/spiOverJtag.bit
+	gzip -9 -c $< > $@
 tmp_%/spiOverJtag.bit : xilinx_spiOverJtag.v
 	./build.py $*